fix(windows): resolve worker pipe typing and POSIX-only test assumptions

Describe the bug

The current source fails Windows-target type checking with seven errors in artifact-processing workers and tests. This is an existing platform qualification issue recorded in RFC #1455's delivery plan, independently actionable before Desktop code is introduced.

The errors concern Windows PipeConnection values passed to Connection annotations and tests referencing POSIX-only os.WNOHANG. These diagnostics establish a type/platform-check failure; they do not by themselves demonstrate a runtime crash.

Steps to reproduce

On Windows, check out aa697c5315204249e090acdf1a00d0582e851c5f, synchronize the locked development environment, and run:

uv sync --locked
uv run --locked --no-sync ty check --python-platform win32

The local reproduction used --exclude docs/my only to exclude untracked personal research files; this exclusion is unnecessary in a clean checkout.

Expected behavior

• Windows connection types are correctly represented for the operations the worker actually uses.
• POSIX-only test behavior is correctly isolated or expressed through a supported platform-specific path.
• Windows-target type checking passes without blanket ignores, removing tests, or relabeling a Linux-target check as Windows validation.
• Add focused Windows type-check CI coverage and run relevant worker cancellation/cleanup behavior tests, preserving Linux behavior.

Actual behavior

Seven errors:

The same command also emitted three url2pathname deprecation warnings in cli/opencode.py; those warnings are separate from these seven blocking errors and do not need to expand this issue's scope.

Environment

• PowerContext source: aa697c5315204249e090acdf1a00d0582e851c5f
• OS: Windows 11 x64, build 26200
• Active environment: Python 3.12.13
• Configured type-check target: Python 3.11, Windows
• ty: 0.0.56
• uv: 0.11.15

Additional context

• Refs #1455 and #1428; delivery-plan platform prerequisite.
• Worker source.
• Test source.
• Existing Windows portability fixes do not eliminate these diagnostics at the inspected commit.
• Open for community implementation as a focused master PR. Desktop scaffolding can proceed in parallel, but these errors must be resolved or correctly platform-scoped before claiming Windows qualification.

Desktop consumer: #1654.

Run the locked Windows-target check with uv and ty, then inspect the connection annotations at src/powercontext/builtin/runtime/artifact_processing.py:203-216 and the os.WNOHANG assumptions in tests/builtin/runtime/test_artifact_processing.py:312 and :382. Confirm the worker cancellation and cleanup tests involved, then add focused Windows type-check coverage. Done means the Windows check passes without blanket ignores while Linux behavior remains covered.