Phase 4a sub-commit 5: performance-smoke harness + 4a closure

examples/todo/tests/perfsmoke.sh asserts the PLAN.md §11 budgets
against the built AppImage:

  - Bundle size ≤ 200 MB (hard cap; ≤ 120 MB target)
  - Cold start ≤ 2000 ms from launch to first /healthz 200
  - Idle RSS (host + descendants in the process group) ≤ 200 MB after
    a 2 s settle.

Each budget is overridable via env (PERF_COLD_START_MS etc.) for slow
shared CI runners; defaults are the strict numbers from the plan. Runs
the AppImage under xvfb-run when DISPLAY is unset; falls back to
QT_QPA_PLATFORM=offscreen otherwise (the build script already bundles
libqoffscreen.so via EXTRA_PLATFORM_PLUGINS).

Wired into:
  - examples/todo/Makefile  → `make perf`
  - .gitea/workflows/release.yml → runs after AppImage build, before
    zsync + upload, with cold-start budget bumped to 4 s for CI.

CI now also installs zsync + xvfb in one step.

examples/todo/README.md gains an "AppImage packaging (Phase 4a)"
section walking through `make appimage`, bundled-mode behaviour, the
auto-update QML hooks (BackendConnection.checkForUpdates() / applyUpdate()),
and `make perf`.

PLAN.md §13 Phase 4 marked **4a closed**. 4b (macOS) and 4c (Windows)
stay stubs until their runners + certs exist.

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

examples/todo/Makefile

@@ -40,6 +40,10 @@ clean: ## Remove build artefacts
 integration: ## Run the bridge-integration test (FrankenPHP boot + HTTP/SSE round-trip + crash-recover)
 	./tests/integration.sh
 
+.PHONY: perf
+perf: ## Run the AppImage perf smoke test (PLAN.md §11 budgets)
+	./tests/perfsmoke.sh build/Todo-x86_64.AppImage
+
 .PHONY: appimage
 appimage: build ## Package as a single-file Linux AppImage at build/Todo-x86_64.AppImage
 	# Composer install --no-dev in a staging copy of symfony so the