`src/engine/bare/compact.rs` — the driver side of compaction

run_compaction and the hook/observer choreography for a Compact step.

Key items

• run_compaction(turn, reason) -> CompactStepOutcome { tokens_before, tokens_after, compacted: Option<Vec<Message>> }:
  1. measure the true size (conversation + overhead — real measurements, never estimates-at-request-time);
  2. no ContextManager → unchanged outcome;
  3. pre-compact hook veto → unchanged outcome (hook guidance merged and threaded when not vetoing);
  4. compact_with_reason(history, turn, reason, instructions, additional_context, reserved) — reserved = overhead + the deferred turn’s transients budget;
  5. Compacted → fire on_compaction + post-compact hooks, return the new messages; NoAction → re-measure, return unchanged; Err(overflow) → log + LoopError::ContextExceeded.

Behavior notes

• The handler feeds compaction_result (rewritten) vs compaction_noop (unchanged) — the machine’s no-progress guard applies to both.
• Observers and post-compact hooks fire only on real compactions — no-op passes are silent.
• Vetoed passes at over-threshold end the run via the guard — documented, deliberate.

Deep dive: Compaction.