bundled: wipe Symfony cache on every launch — mount path bakes into cache

Reproduces with the v0.1.1 AppImage on the second launch (same user
data dir, fresh AppImage mount):

  phpqml.bridge.bundled: symfony:    "/tmp/.mount_Todo-xllnOHH/..."
  Cannot load migrations from "/tmp/.mount_Todo-xDBkOfG/.../migrations"
                                                  ^^^^^^^
                            stale path from PREVIOUS launch's cache

Symfony compiles `kernel.project_dir` (an absolute path) into its
cached container under var/cache/. We redirect var/cache into the
user data dir for read-only-mount survival (v0.1.0 fix), but the
*content* of that cache references the mount path that was active
when the cache was built. Next launch gets a different
/tmp/.mount_<random>; the cached refs are stale; first
project_dir-sensitive lookup blows up (doctrine migrations was the
canary; would also surface as misrouted assets, broken Twig template
paths, etc.).

Fix: BackendConnection::initBundledMode does
QDir(cacheDir).removeRecursively() right after creating the dirs but
before runMigrations spawns the doctrine subprocess. Symfony rebuilds
the cache against the current mount on every launch. Cost: ~1-2s of
warmup per cold start.

Permanent fix is build-time cache warmup (ship the prod cache inside
the AppImage, copy to user data dir on first launch, no per-launch
warmup) — already tracked as a v0.2.0 item in PLAN.md §13. v0.1.1
takes the simpler always-wipe approach since it's bugfix-class.

Regression guard: examples/todo/tests/bundled-supervisor.sh gains a
"2nd launch from fresh staging" step that tears down the first host,
re-stages a fresh fake AppImage layout (different /tmp dir = different
"mount path" from BackendConnection's perspective), and asserts
/healthz comes back up. Without the cache wipe, that step would fail
exactly the way doctrine did in the user's report.

Verified locally:
  - bundled-supervisor.sh passes (incl. 2nd-launch step)
  - Real AppImage: two consecutive launches both reach
    "phpqml.bridge.bundled: migrations OK" + frankenphp spawn

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

examples/todo/tests/bundled-supervisor.sh

@@ -138,4 +138,52 @@ if [ -d "$ROOT/usr/share/$APP_NAME/symfony/var/cache/prod" ] && \
     fail "Symfony wrote into the read-only staging tree — Kernel::getCacheDir override broken"
 fi
 
-step "All bundled-supervisor assertions passed."
+# ── Second launch: same XDG_DATA_HOME, fresh staging mount ─────────────
+# Real AppImages get a fresh /tmp/.mount_<random> per launch but reuse the
+# user data dir, so any cached absolute path from launch N is stale by N+1.
+# Tear down the running host, re-run from a NEW staging dir (mimicking the
+# fresh-mount situation), assert /healthz comes back up.
+step "tear down + relaunch from fresh staging (regression: cache-baked-mount-path)"
+kill -TERM "$PID" 2>/dev/null || true
+for _ in 1 2 3 4 5 6 7 8 9 10; do
+    kill -0 "$PID" 2>/dev/null || break
+    sleep 0.2
+done
+kill -KILL "$PID" 2>/dev/null || true
+PID=""
+chmod -R u+w "$ROOT/usr/share/$APP_NAME/symfony" 2>/dev/null
+rm -rf "$ROOT"
+ROOT="$(mktemp -d)"
+mkdir -p "$ROOT/usr/bin" "$ROOT/usr/share/$APP_NAME"
+cp    "$HOST_BIN"                "$ROOT/usr/bin/$APP_NAME"
+ln -s "$(command -v frankenphp)" "$ROOT/usr/bin/frankenphp"
+cp -a "$STAGING/."               "$ROOT/usr/share/$APP_NAME/symfony/"
+cp    "$CADDYFILE"               "$ROOT/usr/share/$APP_NAME/Caddyfile"
+chmod -R a-w "$ROOT/usr/share/$APP_NAME/symfony"
+
+LOG2="$DATA_DIR/host2.log"
+env -u BRIDGE_URL \
+    XDG_DATA_HOME="$DATA_DIR/share" \
+    XDG_CACHE_HOME="$DATA_DIR/cache" \
+    XDG_CONFIG_HOME="$DATA_DIR/config" \
+    QT_QPA_PLATFORM=offscreen \
+    "$ROOT/usr/bin/$APP_NAME" > "$LOG2" 2>&1 &
+PID=$!
+
+DEADLINE=$(( $(date +%s) + 30 ))
+HEALTHZ2_BODY=""
+while [ "$(date +%s)" -lt "$DEADLINE" ]; do
+    if ! kill -0 "$PID" 2>/dev/null; then
+        sed 's/^/    /' "$LOG2" >&2 || true
+        fail "host died during 2nd boot"
+    fi
+    if HEALTHZ2_BODY="$(curl -fsS -m 1 "http://127.0.0.1:$PORT/healthz" 2>/dev/null)"; then
+        break
+    fi
+    sleep 0.2
+done
+[ -n "$HEALTHZ2_BODY" ] || { sed 's/^/    /' "$LOG2" >&2 || true; fail "/healthz never responded on 2nd launch — stale cache?"; }
+echo "$HEALTHZ2_BODY" | grep -q '"status":"ok"' \
+    || fail "2nd-launch /healthz didn't return status:ok"
+
+step "All bundled-supervisor assertions passed (incl. 2nd-launch cache wipe)."