CLI 3.0 acceptance follow-up: web-as-local: variant and RFC #466 §8 items not exercised #501

Follow-up to the CLI 3.0.0 clean-HOME acceptance (RFC #466 §5). It passed on Windows 11 against 7892c59 built as 3.0.0 with the web provider pinned (web 0.3.1). Some of the RFC's acceptance wasn't exercised:

1. Web as local:. §5 says to run the clean-HOME steps with web as local: first and then pinned. Only the pinned variant was run.
2. §8 acceptance items not checked against the existing test suite:
   • clean-cache toolchain bootstrap with EMSDK unset, PATH scrubbed and LABELLE_HOME empty, asserting the provider provisioned emsdk; the external-EMSDK pass-through; the missing-Python message naming labelle install python;
   • fingerprint/toolchain ordering on a clean cache;
   • cancellation during provisioning, compile and an after build hook, with no orphan (POSIX process group; Windows job object);
   • concurrent toolchain installation;
   • an offline cache miss (LABELLE_OFFLINE=1 with no cached emsdk) failing clearly in web/toolchain.
3. Linux and macOS were only covered by CI, not by a clean-HOME run.

Ask: map each §8 item to an existing test (link it here), or add the missing ones; run the local: variant once.

Evidence from the Windows run (logs, the isolated-env script and the Playwright watch checker) is in the coordinator's C:\prj\accept30.