fix(paseo-omp): preserve Windows process tree during cleanup

[omercnet]

Problem

Windows shutdown started taskkill /T /F and immediately sent stdin EOF. A leader that exits on EOF can disappear before taskkill discovers its ignored-stdio descendant, leaving that descendant orphaned.

TDD regression

The first commit is the real Windows leader-plus-ignored-stdio-descendant reproduction from #151. It requires session.close() to leave no descendant alive. The second commit fixes that failing behavior.

Change

• Preserve concurrent stdin EOF and process-group cleanup on POSIX.
• On Windows, keep stdin open while existing process-tree cleanup runs and when it fails or is uncertain. After verified cleanup, end stdin as a safe exit fallback.
• Add pending-cleanup ordering tests for both OMP RPC and MCP stdio transports.

Verification

• npx vitest run tests/omp-rpc-process.test.ts tests/mcp-transport.test.ts
• npm run typecheck
• npm run check
• npm test
• Windows CI executes the real regression.

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