test fourslash effect mount markers

[mattiamanzati]

Summary

• mount real Effect packages in fourslash tests when @effect-v3 or @effect-v4 markers are present, even without a preexisting from \"effect\" import
• add unit coverage for the mount trigger behavior in etstesthooks
• verify Effect auto-import namespace behavior against the real mounted package for effect/Effect and effect/testing/TestClock

Validation

Passed:

• go test ./etstesthooks -count=1 -v
• go test ./internal/effecttest -run 'TestAutoImportEffectStyleConsistency_(namespace|topLevelNamedReexportsIgnore|topLevelNamedReexportsFollow|testClockWithNamespace)' -count=1 -v

Blocked on current main after pnpm setup-repo:

• pnpm lint
• pnpm check
• pnpm test

Those full-repo commands currently fail due an unrelated generated shim / patched typescript-go sync issue on main, not from this change.