feat(acp): isolate sessions by conversation thread

[eminogrande]

Summary

• Key ACP queues, in-flight state, sessions, affinity, and delivery tracking by (channel_id, thread_root).
• Treat each top-level non-DM post as its own thread session; replies reuse the canonical root; DMs remain continuous.
• Allow different threads in one channel to run concurrently while serializing turns within one thread.
• Label new-session and same-session behavior in the desktop composers.

Context

Addresses #5009 and #3563.

Ports the thread-session direction from #3379 and #2331 onto current main while retaining current per-conversation delivery tracking.

Validation

• cargo test -p buzz-acp: 821 passed
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• All repository just ci stages passed with the pinned Hermit toolchain:
  • workspace and unit suites
  • desktop: 5,122 tests, typecheck, and build
  • Tauri workspace: 2,702 desktop tests plus crate tests
  • web build
  • mobile: 1,552 tests
  • file-size ratchet

No merge performed; upstream review is required.