feat(ui): swap the in-turn loader for a brain-wave sweep left of the send button

Phase 117 (owner request, live-mockup-confirmed): the phase-109 3-dot
cue becomes a compact ECG trace (49px, P/QRS/T) with a brand sweep
traveling the path (bwdraw, 42/140 dash segment, 0.9s loop), the
loader repositioned left of the button so its appearance never shifts
it. setUiState stays the sole owner of the loader's hidden attribute;
the reduced-motion variant stills the sweep. Unit + lifecycle-E2E pins
updated for the new contract.

frontend/index.html

@@ -296,23 +296,37 @@
             autocomplete="off"
             maxlength="4000"
           ></textarea>
+          <!-- Phase 117 (owner request 2026-09-15, live-mockup-confirmed):
+               the persistent in-turn loader — a BRAIN-WAVE: a compact ECG
+               trace (P bump, QRS spike, T bump — 49px wide, 8px flat
+               tails; the owner picked it over the first equalizer mock).
+               .bw-ghost is the always-visible dim trace; .bw-pulse is the
+               brand sweep — a stroke-dasharray segment that travels the
+               path (styles.css bwdraw, pathLength-normalized for a
+               seamless loop). Positioned LEFT of the button on purpose:
+               the row is flex-end aligned with a flex:1 textarea, so the
+               loader's appearance and disappearance eat the textarea's
+               space and the button NEVER shifts (the owner's no-move
+               requirement). The phase-109 D16 contract stands: DECORATIVE
+               (aria-hidden) — #send-status stays the sole a11y announcer,
+               so the state's meaning never moves; hidden by default (idle
+               on load); app.js's setUiState is the SOLE writer of its
+               hidden attribute — shown iff the turn is in flight, hidden
+               by construction in every terminal state (PLAN §7.4 never-
+               stale, no per-handler cleanup). Never constructed in JS: the
+               createElement/textContent house rule — the SVG is static
+               markup too. -->
+          <div id="turn-loader" class="turn-loader" aria-hidden="true" hidden>
+            <svg class="bw" viewBox="0 0 49 32">
+              <path class="bw-ghost" d="M0 16 H8 L15 12 L18 16 L21 18 L24 3 L27 29 L30 16 H35 L38 11 L41 16 H49"></path>
+              <path class="bw-pulse" d="M0 16 H8 L15 12 L18 16 L21 18 L24 3 L27 29 L30 16 H35 L38 11 L41 16 H49" pathLength="140"></path>
+            </svg>
+          </div>
           <button type="submit" class="send-btn" id="send-btn">
             <span class="spinner" aria-hidden="true" hidden></span>
             <span class="btn-label" id="send-label">Send</span>
             <span class="visually-hidden" aria-live="polite" id="send-status"></span>
           </button>
-          <!-- Phase 109 (TODO.md L3, D16): the persistent in-turn loader —
-               ONE static element in the composer's status row, the
-               visible companion of the #send-status line. It is
-               DECORATIVE (aria-hidden): #send-status stays the sole a11y
-               announcer, so the state's meaning never moves (the L1791
-               house pattern). Hidden by default (idle on load); app.js's
-               setUiState is the SOLE writer of its hidden attribute —
-               shown iff the turn is in flight, hidden by construction in
-               every terminal state (PLAN §7.4 never-stale, no
-               per-handler cleanup). Never constructed in JS: the
-               createElement/textContent house rule. -->
-          <div id="turn-loader" class="turn-loader" aria-hidden="true" hidden></div>
         </form>
       </div>
     </div>