W5b-5: Engine object (C11/B9/B12) — single-path, properties resolved at load

[bnsd55]

W5b-5: Engine object (review C11/B9/B12)

load_engine returns a frozen Engine dataclass carrying model, tokenizer, and every per-model property resolved once at load. Strict single-path design per review — no dual (model, tokenizer) path, no optional-profile path.

What ships

• Engine (frozen dataclass): model, tokenizer, model_id (resolved), revision, profile, vocab_size, weight_bytes, cache_capabilities, width_slope. All computed in load_engine; nothing mutates after load.
• Identity: load_engine("quality") and load_engine(full-id) share one cached Engine (lru on the resolved id, unchanged).
• Profile resolved exactly once: _resolve_profile is called from load_engine only; _prefill / _get_or_compute_prior take a required profile parameter fed from engine.profile. Engine.resolve_profile does not exist (zero second path).
• Engine-only entry points: run_parallel_generation(engine, context, schema, ...), run_parallel_generation_batched(engine, contexts, ...), run_naive_generation(engine, ...); calibrate.collect/collect_multi, evalrun.parallel_decide_fn/naive_local_decide_fn, parity.check_scoring_parity/check_batched_parity/parity_report/write_parity_json, serve, bench, and the four benchmark scripts all take an Engine.
• _PRIOR_CACHE key fix: mlx nn.Module models are unhashable (custom __eq__), so weakrefs can no longer live in the key tuple — the key is ids + prompt/plan hashes; the entry keeps the weakrefs and the live-ref check + finalize eviction is unchanged.
• conftest.make_engine: the single factory tests use to build Engines (runs _probe_system_role/_profile_for exactly as load_engine does); test fakes collapsed from (model, tokenizer, ...) to (engine, ...).
• _assemble / _selective_second_pass untouched — no overlap with PR W5-B: shared scalar finalizer, rebuilt dependency second pass, MAP + constraint fixes #45's restructure.

Verification

• Full suite: 690 passed (including all slow real-model tests: engine, smoke, w4b parity, adapters, w1a/w1c).
• ruff: clean on touched files; remaining repo-wide findings are pre-existing UP038 style on main (69 → 17 on this branch).
• Rebased on ecbc77b (PR W5b-11: compiled constraint objects — compile_constraints as the only entry #49 merge; includes PRs W6-1: LM-head adapters module (standalone) #42–W5b-12: docs sweep — ARCHITECTURE.md + CHANGELOG.md for #42/#43/#45/#46/#47/#48 (+ leaderboard v2 follow-up) #52).

[bnsd55]

Rebased over main (4a656e6) and pushed — head 991c454.

• _PRIOR_CACHE key hunk dropped (that code is owned by PR W5-C: exact setcons solver, one count+set optimization, typed CalibrationBundle, internal telemetry, abstention contract #51); the only overlap with W5b-9: CLI error contract + end-to-end smoke suite #54 is a comment reword in _prior_cache_key, which now reads the main-side version.
• cli decide: the _engine() seam is back (loads via the seam so tests can fake it); the engine object flows from there.
• CLI smoke suite fakes load_engine at the seam — fake returns a make_engine() Engine; fake_rpg takes (engine, context, schema).
• Slow tests: all pass except test_api_field_margins_on_real_model, which fails IDENTICALLY on origin/main (KeyError: 'value' at api.py:354; verified in a clean worktree on f9f2ce0) — pre-existing, not from this branch.
• Docs: ARCHITECTURE.md gained an 'The Engine object' section; CHANGELOG updated.

[bnsd55]

Rebased onto 670909e (main, includes #57 + #58 Ledger) — head 7abae06. GitHub now reports MERGEABLE (BLOCKED only by pending checks).

• Ticket refs removed from added lines (engine.py _prefill + load_engine docstrings, conftest make_engine) — sentences kept, codes dropped.
• Ledger plumbing intact: engine.py imports Interval/Ledger and threads the request/per-group/per-context ledgers through the batched path unchanged; test_w5b14_ledger.py passes (14/14).
• Full suite green except test_api_field_margins_on_real_model, which fails identically on origin/main (pre-existing KeyError 'value'; verified earlier in a clean worktree on main).