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>

.gitea/workflows/release.yml

@@ -58,12 +58,24 @@ jobs:
             zsync|${{ github.server_url }}/${{ github.repository }}/releases/download/${{ github.ref_name }}/Todo-x86_64.AppImage.zsync
         run: make appimage
 
-      - name: Generate zsync metadata
-        working-directory: examples/todo/build
+      - name: Install zsync + Xvfb
         run: |
           sudo apt-get update -qq
-          sudo apt-get install -y zsync
-          zsyncmake Todo-x86_64.AppImage -u Todo-x86_64.AppImage
+          sudo apt-get install -y zsync xvfb
+
+      - name: Performance smoke (PLAN.md §11 budgets)
+        working-directory: examples/todo
+        # Cold-start on a shared CI runner is harder than on bare metal;
+        # bump the cold-start budget to 4s here and the deadline to 8s.
+        # The bundle-size and idle-memory budgets stay strict.
+        env:
+          PERF_COLD_START_MS: '4000'
+          PERF_HEALTHZ_DEADLINE_MS: '8000'
+        run: ./tests/perfsmoke.sh build/Todo-x86_64.AppImage
+
+      - name: Generate zsync metadata
+        working-directory: examples/todo/build
+        run: zsyncmake Todo-x86_64.AppImage -u Todo-x86_64.AppImage
 
       - name: Generate latest.json appcast
         working-directory: examples/todo/build