#4164 fix(host-kit): preserve cancellation of live requests

Summary

• Keep every live request abort controller registered until explicit cleanup. Cancellation markers remain bounded.
• Add a 50,002-registration regression and correct ADR 0005: dispatch classification controls mutation replay; command accounting controls runner handoff.
• Three files, two commits (fix, then docs); no scope expansion. Related validation blocker: #4060.

Validation

Base: a0e7c7dc239922bf93adaceff20a2f7e78579146.
Head: d88649c9c82704c4c0f1e9188ffa09b1fa5ec9f3.

• pnpm install --frozen-lockfile && pnpm build: passed.
• pnpm exec vitest run --project unit-core packages/host-kit/src/internal/request-cancel.test.ts: base plus regression 1 failed / 18 passed, early signal remained un-aborted; fixed head 19 passed.
• pnpm format and pnpm check:doc-prose: passed (17 prose tests).
• pnpm check:affected --run --head d88649c9c82704c4c0f1e9188ffa09b1fa5ec9f3: failed. Receipt run-AJzIzW/receipt.json: exactCommit=true, checkoutChanged=false; format, lint, typecheck, layering, Fallow, build passed.
• Related suite: head and isolated unchanged base both 1 failed / 1,226 passed, same record timeout at apple-platform-output-guard.test.ts:385 (#4060).
• pnpm test:unit: 2 failed / 16,964 passed / 1 skipped. Five-second timeouts at committed-source-tree.test.ts:13 and macos-helper-package-contents.test.ts:65; focused head 2 failed / 3 passed, focused base 5 passed. Cause unproven.

Cold self-review only; no findings. No live device evidence. GitHub Integration Tests, Coverage, and platform lanes remain required. Ready for review as requested; validation remains blocked. Scratch copies removed; no timeout changes or skips added.