pin the base image to a digest, with TTL refresh and audit labels

A tag is a moving pointer, and three things conspired to make the base
immutable-in-practice while mutable-in-theory: image_hash() hashed the
string "ubuntu:24.04" rather than what it resolves to, ensure_image()
returns early when the tag exists, and podman build's pull policy is
"missing" -- so even 'build --rebuild' rebuilt on the cached base. A
Canonical patch simply never arrived, and nothing said so.

Now the tag is resolved to a registry digest (skopeo, falling back to a
pull unless AISB_BASE_PULL_FALLBACK=0), recorded per-ref under
$AISB_CACHE_DIR/base/ at 0600, and used in FROM *and* in image_hash():
refreshing the base produces a new image instead of mutating one a
sandbox may be running. Freshness is a TTL (AISB_BASE_TTL_DAYS=7, 0 every
build, -1 never) plus an explicit 'update' / 'update --check'.

Two rules keep it usable rather than merely strict. A stale pin beats a
mutable tag: offline or with AISB_BASE_AUTO_UPDATE=0 the recorded digest
is still used, and only a build with no digest at all is allowed --
announced as UNPINNED, labelled that way, warned about by doctor. And
preview modes stay deterministic: --print-* never reach the network,
inspect is the surface that asks the machine now.

Both images carry aisb.base-digest / .node / .pi-package / .pi-version
labels plus /etc/aisb/manifest.txt of resolved apt versions, which answers
"which squid was enforcing this?" after the build log is gone. Tags are
named after the base instead of the hardcoded u24- prefix.

Bug found on the way: POL[base_digest]="$(base_pin ...)" silently produced
unpinned labels, because an assignment inside a command substitution is
discarded with the subshell. Helpers return text; callers assign. Also
fixed: the manifest step used jq on npm's dependency objects as if they
were strings, and a missing Node checksum used to skip verification
quietly -- it now warns.

311 assertions pass (unit + e2e), shellcheck clean.

LLM-Generated: true

tests/run.sh

@@ -66,6 +66,12 @@ for f in "${FILES[@]}"; do
   (
     export AISB_STATE_DIR="$W/state" AISB_CACHE_DIR="$W/cache" AISB_CONFIG_DIR="$W/cfg"
     export WORK="$W" TESTS="$HERE" AISB_TEST_COUNTS="$COUNTS"
+    # Unit files must not touch a registry: pins come from the seeded cache and the
+    # digest lookup is a stub. e2e files keep the real behaviour.
+    case "$f" in
+      */unit/*) export AISB_BASE_AUTO_UPDATE="${AISB_BASE_AUTO_UPDATE:-0}" \
+                        AISB_BASE_PULL_FALLBACK="${AISB_BASE_PULL_FALLBACK:-0}" ;;
+    esac
     # NOTE: HOME is deliberately not overridden -- podman storage lives there.
     unset AISB_ALLOW AISB_ENV AISB_MOUNT AISB_INSTALL AISB_NET AISB_EXTRA_ARGS AISB_SESSION_OVERRIDE
     source "$f"