Test MCP stdio and terminal commit barriers

[baron]

Summary

• add direct MCPStdioServerTransport tests using controlled pipe file descriptors
• verify complete frames are delivered exactly once and in order before clean EOF
• verify incomplete EOF reports the exact stdinTruncatedFrame byte count
• verify terminal commits reject stale provider-drain generations and resolved revisions are not republished

No production behavior or test-only production seam is changed. The tests introduce no sleeps, retries, polling loops, timeout expectations, child services, or live-app dependencies.

Proof of value

Each test was run against a temporary production defect and failed as intended before the mutation was restored:

• duplicate frame yield: received 4 frames instead of 2
• truncated EOF treated as clean: observed stdinEOF instead of stdinTruncatedFrame(bytes: 16)
• provider-drain guards bypassed: stale request incorrectly published a terminal revision

Validation

• focused stdio suite: 2 tests passed (876bcdfc-4bc0-4b9a-a184-7941b159a439)
• focused terminal-barrier test: passed (bbee632e-387a-4401-9958-17149334d131)
• full root test suite: passed (d22d50f2-5006-4e4b-8642-8daa3d6071c9)
• strict lint: passed (09a6614a-363b-49e1-9742-570929c0e7b7)
• commit and push contribution preflights: passed
• path-selected pr-ready preflight: passed