Simplify queue dispatch and validation

[tmustier]

Summary

• Collapse idle and boundary message handoff into one preparation and delivery path.
• Remove synchronous send and compaction-start recovery branches that Pi's public APIs cannot produce, along with their mock-only tests.
• Remove unused queue methods, unreachable collection guards, stale harness state, and excessive test permutations.
• Replace hardcoded historical validation output with a durable, reproducible validation guide.

Rationale

This is a cleanup follow-up to #9. Pi's extension-facing sendUserMessage is fire-and-forget and converts later rejection into a runtime error event. ExtensionContext.compact reports completion and failure through callbacks. The extension still restores and pauses rows when prompt or Skill expansion fails before handoff, and retains the compaction, reload, image, FIFO, and editing behavior covered by the real integration paths.

Validation

• npm run ci (77 tests)
• ./test/tui-evidence.sh /tmp/pi-queue-steer-followup-evidence-clean
  • Pi 0.84.1
  • clean working tree at 8d7e201
  • manual compaction success and failure
  • native post-compaction ordering
  • automatic overflow compaction
  • two queued runtime reloads
  • abort pause/resume, resource expansion, and all-mode FIFO delivery