`src/compact.rs` — the `ContextManager`

The policy half of compaction: owns the compactor plus window, threshold, target size, and the token counter; classifies every pass.

Key items

• ContextManager::new(compactor) + builders with_context_window (200k), with_threshold (80, clamped 1–100), with_auto_compact (true), with_compact_target (Threshold default), with_compact_target_pct (70), with_token_counter.
• TokenCounter trait + HeuristicTokenCounter — 4 chars/token + 20 chars/message overhead; images 256 flat; deliberately over-estimates (~±30%).
• Entry points: compact_with_reason(messages, turn, reason, instructions, additional_context, reserved_tokens) — the engine’s path; ensure_context_fits (host path, checks its own threshold with >=); compact_manual.
• should_compact(used) / is_emergency(used) / compact_reason(used) — the manager’s own trigger view.
• build_telemetry(...) — for hosts running compaction themselves.

Behavior notes

• Measurements decide, hints describe: the manager re-counts the compactor’s output with its own counter and overwrites the self-reported numbers.
• Classification: same message count and no shrink → NoAction; result over window (minus reserve) → ContextOverflow; else Compacted with normalized figures.
• reserved_tokens (per-request overhead + a deferred turn’s transients) shrinks both the target and the fit check.
• One counter everywhere: the driver reuses the manager’s counter for trigger estimates.

Deep dive: Compaction.