[devsuitup]

Second slice of #1026. The Windows cell now runs only the tests that exercise Windows-specific behaviour. The generic suite keeps running on ubuntu under Node 20 and 22.

• scripts/platform-tests.js (rule C) selects a test when it, or any production module it reaches through relative requires, has a platform marker. Markers cover process.platform, Windows paths, ConPTY, links, real processes and filesystem identity. A test can also opt in explicitly with // platform: windows.
• Tests written for Windows bugs stay selected, and a regression pins them by name: run-to-exit, trigger-continuation-delivery, task-transcript-reader, task-review-regressions, transcript-cwd-trust(-cache), derive-project-path.
• scripts/run-tests.js --platform-only fails closed:
  • a selection under 150 files fails;
  • a selector read error fails;
  • empty stages spawn nothing;
  • outside platform mode, a missing ISOLATED_FILES entry fails.
• test.yml:
  • coverage runs on Linux, platform-only on every other OS;
  • a build-workflow test requires every matrix OS to match exactly one test step;
  • the matrix and the required-check names are unchanged.
• Selection today: 368 of 556 files.

Follow-up, not in this PR: cli-session-state.js pulls in about 43 tests that cannot exercise its platform branch, through bg-worker-liveness.js → procStartMatches. Splitting that pure helper out would cut them.