docs: add ADR for turn-boundary message batching (#598)

* docs: add ADR for batched turn packing in ACP session/prompt

Records the structural decision extracted from RFC #580 (Turn-boundary
message batching): how N concatenated arrival events are packed into
the Vec<ContentBlock> crossing the broker → ACP boundary.

Reuses the existing per-arrival-event <sender_context> template
repeated N times rather than introducing a parallel <message index=N>
wrapper schema, with one additive `timestamp` field on SenderContext.
Closes the attribution gap surfaced independently by Triage (T1.4)
and JARVIS/FRIDAY (B1) reviews.

* docs: supersede batched-turn-packing with turn-boundary batching ADR

Replaces the standalone packing ADR with the consolidated turn-boundary
message batching ADR, which folds RFC #580 mechanism, T1.x dispositions,
and the original packing design into a single document.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* docs(adr): sync turn-boundary-batching ADR with PR #686 Phase 1 implementation

Updates the ADR to match decisions made during PR #686 review:

- 3-valued MessageProcessingMode (per-message / per-thread / per-lane)
  replacing the earlier 2-valued (per-message / batched) design. §4.1
  documents per-mode (cap, dispatcher key, idle timeout) tuple; §4.4 Phase 1
  bullets reflect the unified Dispatcher::submit path; legacy "batched"
  alias is rejected at config parse.

- Standalone <sender_context> Text block (commit 072010c). §3.1 / §3.3 /
  §3.4 / §3.5 / §3.6 + §6.4 rule 4 now describe the split-block layout:
  delimiter + transcripts + prompt + images. Transcripts move from before
  the envelope to inside the arrival event (between delimiter and prompt);
  images stay after prompt as in the pre-batching adapter. Empty prompt is
  omitted from the block stream.

- New §6.10 — per-mode consumer idle timeout (PER_MESSAGE = 10s,
  DEFAULT = 300s) with Little's-Law rationale and sweep_stale eviction.

- New §6.11 — SendError manual staging smoke matrix (the entry deferred
  out of CI in PR #686's first status update).

- §6.7 batch reactions now explicitly sequential (not join_all parallel)
  so reaction-list ordering across a batch matches message-ID order.

- Frontmatter: drop the self-referential "Supersedes: PR #598" line; add
  "Implementation PR: #686" so the ADR points at the wiring it documents.

* docs(adr): fact-check and tighten turn-boundary-batching ADR vs PR #686 head

Five rounds of fact-check + proofread against PR #686
(feature/turn-boundary-batching-v2 @ e119abf) caught two threads of drift:

- Design contract: §2.5 SendError handler now matches commit afd6fff —
  proactive consumer.is_finished() check at submit head + transparent
  retry once on SendError; ❌ + ⚠️ + Err(ConsumerDead) only if the retry
  also fails. Motivated by the first-message-after-idle race; one-attempt
  bound preserves the no-spin-loop property. §6.11 staging smoke matrix
  split into Path A (PANIC_ONCE happy path, no user-visible signal) and
  Path B (PANIC_ALWAYS failing-retry surfaces ❌+⚠️). §4.4 Phase 1 plan +
  test list updated to the new contract.

- Anchor audit vs declared base v0.8.2-beta.1 (52052b8): pre-existing
  drift fixed in adapter.rs references that had been wrong since the SHA
  pin was set in v0.2 — :131-152→:156-172, :138-143→:158-162 (7 sites),
  :148-152→:165-169, :154-161→:173-180, :181→:254 (was pointing at the
  wrong call), :240→:260. acp/connection.rs / acp/pool.rs / discord.rs /
  slack.rs anchors verified clean against 52052b8.

- §2.6 rewritten: other_bot_present is a bool snapshot carried on
  BufferedMessage and read from batch.last() at dispatch time — not the
  Arc<AtomicBool> mirror of an earlier draft. §2.3 struct + submit
  signature corrected to match.

- Anchor-pinning preamble (line 9) expanded to pin both SHAs explicitly:
  released-code anchors → 52052b8; conceptual descriptions of new
  modules → cross-checked against e119abf.

- Appendix A replaced with a signatures-only skeleton pointing at
  src/dispatch.rs — drops the ~200-line body sketch that had drifted
  from the implementation; rationale moved into a short shape-choices
  list.

- Path anchors swept: pool.rs → acp/pool.rs, connection.rs →
  acp/connection.rs (modules live under src/acp/ in v0.8.2-beta.1).

- §6.6 metric table cell tokens_per_event (was context_tokens_per_event,
  inconsistent with the code block immediately below).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Brett Chien <1193046+brettchien@users.noreply.github.com>

Author: brettchien