Adds W1-T: an observational telemetry leaf for idle_tick's learning flywheel,
modeled on core.cognition.leeway (the B4 precedent). Never gates, refuses,
or alters trace_hash.
New core/cognition/backpressure.py:
BackpressureRecord(pending_proposals, candidate_backlog, cap, headroom,
contemplated_this_tick, created_this_tick, at_fixed_point, did_work)
build_backpressure_record(...) — pure function, resolves ADR-0161 cap from
env (CORE_HITL_PENDING_CAP or 256), derives headroom = max(0, cap-pending),
at_fixed_point = (candidate_backlog==0 and created_this_tick==0).
chat/runtime.py:
IdleTickResult gains backpressure: BackpressureRecord | None = None (always
set on a completed tick).
idle_tick builds the record from counts already computed (pending_proposals,
candidate_backlog, contemplated_count, created, did_work) — no new
computation, no serving-path change.
Appends one JSONL line per tick to engine_state/idle_telemetry.jsonl
(best-effort, never crashes the continuous life; accumulates across all
generations so history survives reboot independently of the checkpoint).
tests/test_idle_backpressure_telemetry.py (16 tests):
- field derivation: headroom, cap, at_fixed_point (holds + bites)
- cap resolution: default 256, env override, invalid env fallback
- firewall proof: trace_hash of a served turn is byte-identical whether
idle_tick ran before it or not (the load-bearing invariant)
- integration: record fields on a fresh runtime; telemetry written to
engine_state dir; history accumulates across ticks
All 16 tests pass. ADR-0161 cap and queue_full control logic untouched.
