fix(test): make ecosystem imports and shutdown checks portable

[jimhoyd]

Problem and change

Run 36886904873 fails Windows Node 24 packed ecosystem integration with ERR_UNSUPPORTED_ESM_URL_SCHEME: import() receives a bare Windows drive path. Convert the installed module path with pathToFileURL before importing it.

The test also assumed SIGTERM invokes Node handlers on Windows. Add a separate explicit close() check on every platform, requiring natural process exit and port release. Keep the signal-handler assertion on Unix and explain the Windows limitation in the fixture README. All application, sandbox and host-admission assertions remain intact.

Fixes #1128.

Verification

• npm run test:ecosystem: all 13 tests passed locally.
• npm run verify: passed, including 1,397 root tests, workspace verification and application proofs.
• All PR checks passed.
• Full matrix: Windows integration is still waiting for upstream package jobs. A separate Windows Node 24 test/prepare.test.ts npm-pack subprocess timed out after emitting archive JSON; tracked in [Bug]: Windows npm pack fixture times out after emitting archive JSON #1130, cause unverified. The full matrix is therefore not green.

Compatibility and security

Integration-test and evidence-documentation changes only. No runtime, public API, policy or sandbox changes. Windows process termination is not presented as graceful runtime cleanup.

• Changes match the documented portable contract; docs/examples are updated where needed.
• Relevant tests and package checks pass, or limitations are explained above.
• No credentials, customer data or private project material are included.
• Runtime/starter changes remain aligned where applicable.