Re-flush mailbox mail from the fleet stall poll

[TheGreatAxios]

Summary

• The periodic fleet stall poll re-flushes mailbox mail, so a missed subscribe-time wake self-heals within one poll interval
• Both poll halves stay no-ops when there is nothing to deliver: the fleet report diffs and the flush no-ops while the parent is processing or no uncollected terminal waits

Verification

• bun run typecheck passes; bun run lint and bun run build pass as part of bun run check
• New src/tui/runner/wiring.stall-poll.test.ts (3 tests) passes, covering a missed driver edge, a mid-turn terminal, and no duplicate delivery
• Related suites pass: runtime-bridge, mailbox-mail-drive, wiring.ask-wake (87 tests)
• Full bun run check suite: 6708 pass with 4 failures in assemble-runtime and optimized-context-store that also fail on clean origin/main, so they are pre-existing and unrelated

Fixes CL-7676

[linear-code]

CL-7676