[AZ-301] Implement EngineGate — D-C10-3 + D-C10-7 takeoff validator

AZ-301 takeoff-side validator every InferenceRuntime strategy calls
before deserialize_engine. Five-step deterministic refusal pipeline,
in order:

  1. filename schema parse  -> EngineSchemaMismatchError(reason=...)
  2. schema tuple match     -> EngineSchemaMismatchError(expected,got)
  3. sidecar present        -> EngineSidecarMissingError
  4. sidecar trust          -> EngineHashMismatchError(stage=sidecar)
  5. manifest match         -> EngineHashMismatchError(stage=manifest)

Refusal order is part of the public contract (AC-7 verifies a
fixture that is BOTH schema-mismatched AND missing-sidecar refuses
at step 1).

Production code (new):
 - components/c7_inference/engine_gate.py  -- EngineGate, HostTuple,
   read_host_tuple (Jetson: pynvml + /etc/nv_tegra_release +
   tensorrt.__version__; raises RuntimeError on Tier-1)
 - components/c7_inference/manifest.py     -- DeploymentManifest,
   ManifestReader, ManifestReaderProtocol. Risk-2 enforced at the
   type level: __getitem__ raises EngineHashMismatchError on
   missing key, NEVER KeyError, so the gate cannot silently pass
 - components/c7_inference/__init__.py     -- re-exports the new
   public surface

Tests (new): tests/unit/c7_inference/test_engine_gate.py covers
AC-1..AC-7 + NFR-reliability-no-write + manifest reader + refusal
log emission. 14 tests unconditional + AC-8 Tier-2 skip (needs
real NVML + L4T release file + tensorrt binding).

Three task-spec -> as-built deltas documented in
_docs/02_tasks/done/AZ-301_c7_engine_gate.md Implementation Notes:
 1. HostTuple lives in engine_gate.py (the only consumer);
    re-exported from package __init__.py.
 2. read_host_tuple takes precision as a keyword argument — three
    of four fields come from the host, precision is engine-build
    metadata supplied by the caller.
 3. AC-8 is Tier-2-only; AC-1..AC-7 + NFR-reliability + extras
    run on every CI host.

Risk-2 (manifest reader silently treats missing entry as pass):
DeploymentManifest.__getitem__ raises EngineHashMismatchError with
"missing manifest entry for {path}" — covered by
test_manifest_missing_entry_raises_hash_mismatch.

NFR-perf-validate (p99 <= 50 ms): tier-2 only — a real 500 MB
engine streaming sha256 cannot be benchmarked on Tier-1 fixtures.

AZ-302 (ThermalStatePublisher) + AZ-304 (C6 Postgres schema)
deferred to batches 26 / 27 to keep the 1-task batch cadence and
isolate their respective env / testcontainer surface areas.

Suite: 1134 passed / 11 skipped. No regressions outside the new
files.

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/02_tasks/done/AZ-301_c7_engine_gate.md

@@ -159,3 +159,24 @@ Then the returned tuple has `sm=87, jp="6.2", trt="10.3"`; on a workstation (Tie
 - **Production code that must exist**: real `EngineGate.validate` calling real `helpers.engine_filename_schema.parse`, real `helpers.sha256_sidecar.verify`, real `ManifestReader` reading the deployed manifest.json from disk.
 - **Allowed external stubs**: tests MAY inject a `dict`-backed `ManifestReader` (AC-3..AC-7); production wiring reads the on-disk manifest.
 - **Unacceptable substitutes**: a "warn-only" mode that logs but does not raise (would defeat the safety gate); a manifest reader that silently treats missing entries as pass (covered by Risk 2 mitigation); a fast-path that skips sidecar verification when the manifest is present (would weaken D-C10-3 against an attacker who tampers with the engine file post-deploy).
+
+## Implementation Notes (2026-05-12, batch 25)
+
+Three minor task-spec → as-built deltas:
+
+1. **`HostTuple` lives in `engine_gate.py`** — spec said "`HostTuple` dataclass and a stateless `read_host_tuple()` helper" but didn't pin a module. Co-located with the gate (the only consumer); re-exported from `c7_inference` package `__init__.py`. Future consumers can lift it out if needed.
+
+2. **`read_host_tuple()` requires explicit `precision` argument** — the helper queries NVML for `sm`, `/etc/nv_tegra_release` for `jp`, `tensorrt.__version__` for `trt`, but `precision` is engine-build metadata, not a host property. Caller passes it. Spec implied that — the tuple "derived from `nvidia-smi`/`pynvml` + the runtime's pinned TRT version + the engine's intended precision (read from the entry)".
+
+3. **AC-8 is Tier-2-only** — marked `@pytest.mark.tier2 + @pytest.mark.skipif(GPS_DENIED_TIER!=2)`. The helper needs real NVML + `/etc/nv_tegra_release`, neither of which exists on macOS / Tier-1 Linux. AC-1..AC-7 + NFR-reliability + manifest-reader coverage run unconditionally (14 tests).
+
+### As-built file map
+
+- `src/gps_denied_onboard/components/c7_inference/engine_gate.py` — `EngineGate.validate`, `HostTuple`, `read_host_tuple` (+ `_read_jetpack_version`, `_read_tensorrt_version`, `_sha256_of_file` private helpers).
+- `src/gps_denied_onboard/components/c7_inference/manifest.py` — `DeploymentManifest`, `ManifestReader`, `ManifestReaderProtocol`. Missing-entry access raises `EngineHashMismatchError` (NOT `KeyError`), per Risk 2.
+- `src/gps_denied_onboard/components/c7_inference/__init__.py` — re-exports `EngineGate`, `HostTuple`, `DeploymentManifest`, `ManifestReader`, `ManifestReaderProtocol`.
+- `tests/unit/c7_inference/test_engine_gate.py` — 15 tests (14 unconditional + AC-8 tier-2 skip).
+
+### Refusal-order discipline
+
+The five steps execute in this exact order; AC-7 verifies the property by passing a fixture that is *both* schema-mismatched and missing-sidecar — the schema error wins because step 1 runs first. Future refactors that reorder the steps regress AC-7.