[mike-doctorina]

Stacked on #27, merge after it.

Problem

On the first load after a deploy, the previous build's worker still controls the page:

• it serves its cached main.dart.* cache-first;
• index.html (network-first) and bootstrap.js (never intercepted) are already the new build's;
• the bootstrap loads CanvasKit or skwasm for the new engineRevision.

On the same engine that mix runs, because flutter.js and the engine only change together with the Flutter SDK. After a Flutter upgrade it fails inside the engine (WebAssembly.instantiate(): Import #N). On a live deployment the same first load also waited out the full 4 s activation timeout before CanvasKit started.

Fix

• Worker: reports its build's engineRevision in the getVersion reply. The generator writes it into the sw.js config, and only when the build has one.
• Bootstrap (settleUpdateAtBootstrap, runs after registration and outside the 10 s registration cap):
  • It asks the controller for its engine revision, waiting at most 500 ms.
  • Same engine: the page boots with the current controller at once and the new worker takes over on the next load. registerAndActivate no longer waits 4 s for a worker that cannot activate while the page is open.
  • Different or unknown engine (workers from earlier releases don't report it, so the first load after upgrading takes this path once):
    • It waits up to 60 s (SW_UPDATE_INSTALL_TIMEOUT_MS) for the new worker to install. The precache progress is on screen through the existing sw-progress counter.
    • It then hands control to the new worker before any app file is requested.
    • If the handover is lost, it reloads once, with a sessionStorage guard against loops.
  • Failed or timed-out install: the page boots with its current controller, as it did before.

Verification

• Playwright (e2e/engine-update.spec.ts, new): deploys the example twice over a local host, with a marker in main.dart.js.
  • Engine changed: the first load after the second deploy runs the new build. With the bootstrap's call to settleUpdateAtBootstrap removed it runs the old one (Expected "b", Received "a").
  • Same engine: the page boots with the old build at once and reaches the new one on a later load.
  • The new spec passed 10/10 with --repeat-each=5. The full suite passed, 19/19, run locally on installed Chrome.
• Vitest: 303 pass, 12 of them new. 6 of the settleUpdateAtBootstrap tests fail against a stub.
• Dart: 115 pass, integration included. analyze, format and npm run verify are clean.

Overlap with #25

This includes #25's first-load change: a controlled page no longer waits for a pending worker. It does not include #25's shorter 500 ms handover bound. When #25 is rebased, its registerAndActivate part reduces to the handover timeout.

[mike-doctorina]

Superseded by #31, which carries these commits unchanged plus the fixes for #30.