fix(windows): make worker pipe typing portable

[Doar999]

Which issue or RFC does this PR close?

Closes #1658.

Rationale for this change

Windows spawn pipes are represented as PipeConnection, while the worker lifecycle annotated them as POSIX Connection objects. The lifecycle tests also depended on POSIX-only process APIs and spawn internals, causing seven Windows-target type-checking errors and preventing the cleanup behavior from being exercised on Windows.

Related work and acknowledgement

Thanks to @knqiufan for sharing the overlapping draft implementation in #1676 and closing it in favor of the existing community contribution.

I reviewed that draft as an optional reference. This PR uses capability-based pipe protocols and platform-specific spawn seams so the existing cleanup tests can run on Windows instead of being skipped. It also incorporates the focused Windows CI structure and retains coverage for cancellation after a child has been published.

What changes are included in this PR?

• Model the worker pipe endpoints with separate sender and receiver protocols exposing only the operations each endpoint uses.
• Select the appropriate spawn interception seam on each platform: Popen._launch on POSIX and Popen.__init__ on Windows.
• Add platform-specific child exit and reaping assertions.
• Treat only ERROR_INVALID_PARAMETER as evidence that a Windows PID no longer exists; propagate other OpenProcess failures.
• Run the existing worker startup, timeout, cancellation, pipe cleanup, and pre-publication failure tests on Windows.
• Add regression coverage for cancellation after the child process has been published but before the start operation returns.
• Add focused Windows-target type checking and worker lifecycle testing to CI.

Are there any user-facing changes?

No public API, schema, dependency, configuration, or persisted-format changes.

The change improves Windows type safety and validates worker cleanup behavior without changing the worker lifecycle contract.

How was this change tested?

• Windows-target type checking:
  uv run --locked --no-sync ty check --python-platform win32 src/powercontext/builtin/runtime/artifact_processing.py tests/builtin/runtime/test_artifact_processing.py
• Linux-target type checking:
  uv run --locked --no-sync ty check --python-platform linux src/powercontext/builtin/runtime/artifact_processing.py tests/builtin/runtime/test_artifact_processing.py
• Actual Windows worker lifecycle suite: 7 passed
• Ruff lint and format checks: passed
• Workflow YAML validation: passed
• GitHub Actions reference validation: 79 references checked

The repository-wide Windows type check also reports three pre-existing url2pathname deprecation warnings that #1658 explicitly identifies as out of scope. The focused Windows check added by this PR passes without ignores or warning suppression.

AI usage statement

OpenAI Codex (GPT-5) was used to inspect the issue and review feedback, implement the fixes, design regression tests, and review the final change.