Make interrupted browser bundling cleanup reliable under the concurrent verifier

[taras]

Problem

The exact concurrent verification battery can leave the Deno browser bundler output in a test-owned scratch directory after the Effection scope has torn down.

Observed on commit b82121c while verifying PR #415 on macOS arm64 with Deno 2.9.5:

deno task verify
verifying 9 commands concurrently (site/ unchanged since origin/main), 20m deadline each

ok      vendor     4.5s
ok      lint       2.7s
ok      check      1.6s
FAILED  test       792.8s (exit 1)
ok      check:jsr  1.5s
ok      tsc        22.9s
FAILED  test:node  271.8s (exit 1)
ok      test:bun   652.5s
ok      docs       20.3s

build-web-client ... leaves nothing behind when a build fails
Expected: []
Received: ["2361ba29aac95f1f.js"]
at scripts/tests/build-web-client.test.ts:237:37

FAILED | 424 passed (2971 steps) | 1 failed (1 step) (13m11s)

The failure is in the test that starts buildWebClient({ scratch }), ends its scoped owner with the check that started the build failed, and then requires the scratch directory to be empty. The lingering hash-named JavaScript file is the real deno bundle output.

The identical committed revision immediately passed the isolated Deno file:

deno task test scripts/tests/build-web-client.test.ts
ok | 3 passed (24 steps) | 0 failed (14s)
leaves nothing behind when a build fails ... ok (57ms)

The same verifier invocation also reported a Node runner failure with no second-failure detail. The identical revision then passed pnpm test:node alone with 2,708/2,708 tests in 200.3s; that phantom runtime failure is recorded on #275.

Concurrency evidence

• Failure: the full nine-command battery, with Deno, Node, Bun, builds, typechecks, and publishability running together.
• Pass: the identical Deno test file in isolation without a code change.
• Pass: the identical full Node suite in isolation without a code change.
• The branch changes only Workspace coordination source, tests, and specifications; it does not change the browser build or process adapter.

Acceptance

• The exact concurrent deno task verify battery repeatedly leaves no hash-named bundle output after either interrupted-build test.
• Scope teardown joins or terminates the real bundler before scratch cleanup completes.
• The regression uses lifecycle barriers or process ownership, not sleeps, polling, retries, a longer verifier deadline, serialization, or an exclusion.
• The isolated and concurrent Deno/Node/Bun suites remain green.