fix(chat): keep in-flight answers alive across in-app view switches

Root cause (owner repro, verified in a real browser 2026-09-06): the
five navbar views (Chat, RAG, Sources, Tuning, History) were separate
HTML documents, so a navbar click was a REAL cross-document navigation
— the chat page unloaded, the in-flight SSE fetch was aborted, and the
phase-48 teardown (app/api/chat.py `finally`, "chat: turn cancelled")
stopped the model. Observed: send question -> click RAG mid-stream ->
click Chat -> the answer never finished: no `query_log` row, and on
return a dangling question with no brain record (the pre-token pagehide
partial persist skips because `acc` is empty).

Phase-48 LOCKED-DECISION REFINEMENT (owner-confirmed 2026-09-06,
flagged per AGENTS.md rule 3, not silently deviated): "real navigation
cancels the fetch" now means LEAVING THE APP — tab close,
external/other-document navigation, the Stop button. In-app navbar
switches are client-side view switches and no longer cancel.

Fix — Option A (SPA shell), chosen over B (Service Worker owns the
stream) and C (server-side turn registry + resume):
- frontend/index.html is the shell: ONE `<main id="main">` holds the
  five `<section class="view">` blocks; hidden views carry BOTH
  `hidden` and `inert` (WCAG — no focus/keyboard traversal). The
  shared header, the single `doc-modal-*` skeleton, and the
  `#app-version` footer each exist exactly once; the per-view copies
  from the four folded pages are dropped.
- New frontend/assets/router.js (vanilla module — no framework, no
  bundler, No-CDN rule intact): lazy-imports a view module on FIRST
  show only (mount-once, hide-forever — the chat view's in-flight SSE
  reader persists across switches; that persistence IS the fix);
  intercepts same-shell navbar links with preventDefault +
  history.pushState (never a document load); handles popstate; single
  writer of `.nav-link` active state (is-active + aria-current),
  document.title, and the per-view meta description (values carried
  over from the old pages' heads, brand-resolved at write time).
- Each folded page's JS becomes `export async function mount(root)` —
  root-scoped queries; `initSharedHeader()` dropped (the header boots
  once in the shell via the chat module; the admin flag comes from the
  same cached `fetchIsAdmin()` promise — zero extra requests).
- app/main.py: a small list-driven route factory serves the shell for
  /tuning.html, /sources.html, /git-sources.html, /history.html —
  registered AFTER the API routers and BEFORE the static catch-all
  (routes-first). The phase-33 caching middleware applies no-cache +
  `?v=` rewriting unchanged; app/core/caching.py needed NO change
  (the view paths did not change — pinned by the integration tests).
- The four old view .html files are DELETED (one source of truth);
  deep links to the old URLs keep working (the router picks the view
  from the pathname); `/?chat=<id>` is unaffected; the Containerfile
  bundles router.js (inlining the lazy view modules) and drops the
  folded page files.
- app/schemas.py: HistoryTurn.text cap 4000 -> 32000 — the shell
  keeps long saved answers in the chat, and the old cap (stricter than
  the 24_000-char total history budget) 422-rejected any second turn
  in such a chat (found by the phase-42 E2E suite on the shell).

Boundaries: login.html, shared.html, doc-edit.html, document.html
REMAIN separate documents (flow pages, not navbar tabs); a mid-stream
navigation to doc-edit/document.html still cancels per phase 48
(follow-up candidate, out of scope). The SSE API is unchanged. Real
departures still cancel the turn — phase 48 intact (pinned by
tests/e2e/test_stop_generation.py, unchanged, and by the new suite's
real-departure control).

Tests:
- Phase-20 suite REWRITTEN to the new semantics
  (tests/e2e/test_sources_midstream_bug.py): a navbar switch no longer
  cancels — the stream survives the switch and the FULL answer
  settles; the pagehide partial persist REMAINS for real departures
  (the partial's exact shape — first streamed chunk prefix, no done
  metadata — is still pinned there).
- NEW story suite tests/e2e/test_nav_switch_keeps_stream.py (mock
  LLM): the owner repro (send -> RAG mid-stream -> Chat: window
  sentinel survives = same document, FULL answer, exactly one brain
  turn in bor.chat.v1, exactly one settled query_log row, auto-saved
  row matches) + the same mid-stream switch against the other three
  views + the real-departure-still-cancels control + the no-switch
  baseline.
- tests/unit/test_frontend_router.py: source-level pins of the router
  invariants (click interceptor targets ONLY same-shell view paths,
  pushState-only switches, mount-once guard, hidden+inert pair,
  single-writer active state/title); shell-route integration tests
  (each folded path serves the shell with no-cache + `?v=` body; a
  non-view path still 404s); the file-reading unit pins re-pointed at
  the shell (the four view files are gone — the shell is the source
  of truth).

Verification (this commit): full suite green — 1565 unit+integration
tests, app/ coverage 99% (>90% floor); ruff + pyright clean; the
phase's E2E suites green in isolation (house protocol, AGENTS.md rule
9). Owner repro verified in a real browser against the real LLM
(dev server :8010, headful Chromium): "tell me about everquest" ->
RAG mid-stream -> Chat — the answer completed with one brain bubble
and no error banner, `query_log` gained exactly one settled row
(deflected=True: the dev KB holds no EverQuest docs — the settle, not
the topic, is the proof), zero "chat: turn cancelled" lines for that
turn; the control (real navigation to /shared.html mid-stream) still
cancelled (no settled row, the cancel line logged, the partial
persisted on return). Screenshots: .agents/screenshots/76_manual_*.

Phase 76 (76_spa_nav_shell) complete — moved to
.agents/phases/complete/.

tests/unit/test_history_copy.py

@@ -1,11 +1,18 @@
 """Unit: phase-66 text pins — the History tab describes the auto-save
 model, not the retired Save button.
 
-House pattern (``test_stale_ui_copy.py``): read ``frontend/history.html``
-as text and assert substrings — no browser. The browser-visible layer is
+House pattern (``test_stale_ui_copy.py``): read the frontend files as
+text and assert substrings — no browser. The browser-visible layer is
 gated by the dedicated story suite (``tests/e2e/test_history_copy.py``);
 these pins catch a silent regression in the template without it.
 
+Phase 76 (task 03): the History view is a view of the shell —
+``frontend/history.html`` is deleted, the view copy lives in the
+shell's ``#view-history`` section, and the per-view meta description is
+the router's (``frontend/assets/router.js`` — the single writer of the
+client-side ``<meta name="description">``, value carried over from the
+old page's ``<head>``). The pins follow the copy to its new home.
+
 Locked decision (owner-locked A3, 2026-09-01): every conversation saves
 itself automatically — there is NO Save button (retired phase 55, owner-
 locked A2, pinned by ``test_save_share_ux.py::test_anonymous_auto_save``),
@@ -18,6 +25,9 @@ from pathlib import Path
 
 FRONTEND = Path(__file__).resolve().parents[2] / "frontend"
 
+SHELL_HTML = FRONTEND / "index.html"
+ROUTER_JS = FRONTEND / "assets" / "router.js"
+
 # --- the locked auto-save copy (owner-locked A3) -----------------------
 
 META = "Saved chats — every conversation is saved automatically, one click back."
@@ -43,8 +53,24 @@ STATE_STRINGS = (
 )
 
 
-def _text() -> str:
-    return (FRONTEND / "history.html").read_text(encoding="utf-8")
+def _shell() -> str:
+    assert SHELL_HTML.is_file(), f"missing {SHELL_HTML}"
+    return SHELL_HTML.read_text(encoding="utf-8")
+
+
+def _router() -> str:
+    assert ROUTER_JS.is_file(), f"missing {ROUTER_JS}"
+    return ROUTER_JS.read_text(encoding="utf-8")
+
+
+def _view(html: str) -> str:
+    """The shell's History view section (the view is the shell's LAST
+    view section — the slice runs to the container main's close)."""
+    start = html.find('<section class="view" id="view-history"')
+    assert start != -1, "the #view-history section must be in the shell"
+    end = html.find("</main>", start)
+    assert end != -1
+    return html[start:end]
 
 
 def _norm(text: str) -> str:
@@ -54,26 +80,32 @@ def _norm(text: str) -> str:
 
 
 def test_manual_save_copy_is_gone() -> None:
-    """history.html: the three retired manual-save strings are GONE —
-    the copy no longer tells the visitor to press a Save button."""
-    html = _text()
-    for frag in (OLD_META, OLD_PAGE_SUB, OLD_EMPTY_ROW):
-        assert frag not in html, f"retired manual-save copy still present: {frag!r}"
+    """The shell (which carries the History view) and the router (which
+    carries the per-view meta): the three retired manual-save strings
+    are GONE — the copy no longer tells the visitor to press a Save
+    button."""
+    for text in (_view(_shell()), _router()):
+        for frag in (OLD_META, OLD_PAGE_SUB, OLD_EMPTY_ROW):
+            assert frag not in text, f"retired manual-save copy still present: {frag!r}"
 
 
 def test_locked_auto_save_copy_present_exactly_once() -> None:
-    """The three locked (A3) auto-save strings, each exactly once in
-    history.html (a second copy could drift out of sync)."""
-    html = _norm(_text())
-    assert html.count(META) == 1, "the locked meta description"
-    assert html.count(PAGE_SUB) == 1, "the locked page-sub"
-    assert html.count(EMPTY_ROW) == 1, "the locked empty-row string"
+    """The three locked (A3) auto-save strings, each exactly once (a
+    second copy could drift out of sync). Phase 76 (task 03): the meta
+    description lives in the router's DESCRIPTIONS table (the router is
+    the single writer of the client-side meta — the value is carried
+    over from the old page's <head>), and the page-sub + empty-row
+    strings live in the shell's History view."""
+    assert _router().count(META) == 1, "the locked meta description (router-owned)"
+    view = _norm(_view(_shell()))
+    assert view.count(PAGE_SUB) == 1, "the locked page-sub"
+    assert view.count(EMPTY_ROW) == 1, "the locked empty-row string"
 
 
 def test_state_language_survivors_are_untouched() -> None:
     """Proof of NO over-deletion: the strings where "saved" is a state —
     the <h1>, the anonymous gate title, and the gate sub — stay exactly
-    as phase 50 wrote them."""
-    html = _text()
+    as phase 50 wrote them (in the shell's History view)."""
+    view = _view(_shell())
     for frag in STATE_STRINGS:
-        assert frag in html, f"state-language survivor deleted: {frag!r}"
+        assert frag in view, f"state-language survivor deleted: {frag!r}"