Run the chat-app Web smoke in Safari and Firefox and add a WebGPU generation E2E (#1025) - #2002

Summary

• run_local_e2e.dart --scenario chat-app-web-real-model-smoke --browser safari|firefox serves the repo with --coep require-corp and drives the smoke through safaridriver or geckodriver (FIREFOX_EXECUTABLE, GECKODRIVER).
• --backend webgpu passes --backend-index 0 --gpu-layers 99 (default stays CPU Wasm).
• The Dart web scenarios reject --browser safari|firefox: package:test pages are never cross-origin isolated.
• New web-webgpu-generation scenario and test/e2e/webgpu/webgpu_generation_e2e_test.dart (local-only): full-offload WebGPU text vs CPU Wasm, direct and worker.
• Firefox injects the init script as a page <script> because geckodriver's execute sandbox hides page globals.
• --backend webgpu now also passes --expect-gpu-active true (a CPU fallback fails) and, with --backend, only cpu or webgpu are accepted by this scenario (stricter allowlist); --backend webgpu is rejected on other scenarios. Safari and Firefox run Gate Web ASR runtime and microphone lifecycle in the harness (#329) #1999's runtime probe.
• Firefox page-state reads run in the page world, like the init script.
• Supersedes Run the chat-app Web smoke in Safari and Firefox and add a WebGPU generation E2E (#1025) #2000 (rebased on Gate Web ASR runtime and microphone lifecycle in the harness (#329) #1999; the head-update tool cannot fast-forward a rebase).
• Part of #1025, plan #1995. Support wording lands with the repin PR.

Production-readiness scope

• User-facing scope: maintainer test harness only; no lib/ change.
• Supported platforms/paths: macOS Safari 27, Firefox via geckodriver, Chrome WebGPU.
• Unsupported or intentionally unavailable paths: Safari/Firefox on Dart web scenarios exit 64 with the isolation reason; tool-call, speech and mmproj checks in Safari/Firefox exit 2.
• Out of scope / follow-ups: Firefox real run (see Matrix Evidence); coordinates with #1999, which edits the same four files in separate hunks.

Completeness checklist

• Declared scope is fully implemented, or explicitly reduced above.
• Unsupported platform/option combinations fail loudly with actionable diagnostics or are clearly documented as unavailable.
• Public API docs, README/website docs, examples, support matrices, and changelog entries are updated where relevant. (N/A: no public API; matrix doc updated; changelog and support docs belong to the repin PR)
• Regression coverage covers the original issue plus key negative/version-skew paths where relevant.
• Public capability changes update the validation coverage registry. (N/A: none)
• Security/privacy review completed: no secrets leak.
• Follow-up work tracked, or "None" above.

PR type guidance

• Feature PR: N/A, test tooling only.

Test Plan

• dart run tool/prepare_workspace.dart
• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test -p vm test/unit/tooling/ (1830 passed); full -j 1 VM and Chrome suites left to CI
• coverage-lib: N/A, no lib/ change
• Other: mutation check (dropping --coep require-corp fails the dry-run and matrix tests); real runs below

Matrix Evidence

Review Notes

• Independent review status: pending (high-risk)
• CI status / head SHA: pending, pending

High-risk regression review

• Classification: high-risk (regressionPolicy)
• Implementation task: this PR
• Independent blocking QA task: pending
• Exact head / current base: pending
• Production-branch deletion, bypass, or miswire proof: N/A
• Affected-family real-model/artifact evidence: Qwen3.5-0.8B (Safari), stories15M (Chrome generation)
• Explicit unavailable-family or other N/A evidence: Firefox real run
• Known PR-caused P1 regressions: 0
• Unresolved review threads: 0