Exposes the Foundry's mechanical loop-quality grader (the "katana") on
/audio/v1/grade: upload a loop/one-shot → composite 0..1 + S/A/B/C/D tier,
per-rule sub-scores (seam click, zero-crossing cleanliness, DC, bar
self-consistency, level, bass mono-compat) + human-readable flags. scope `grade`,
cached, torch-free. Directly serves "iterate on sampling quality".

engines/grade.py is a BYTE-FOR-BYTE vendored copy of tools/foundry/engine/grade.py
(self-contained — numpy/soundfile/pydantic/librosa, no Foundry/torch — because the
container holds only armada/api/). It's kept identical on purpose, and
tests/test_grade_endpoint.py is a DRIFT GUARD: it imports the Foundry canonical
standalone and asserts the vendored copy grades identically (grade/tier/sub +
WEIGHTS/THRESH) on a synth signal — a future Foundry tweak that isn't re-vendored
fails CI here, not silently in prod (parsers-over-copy, applied to a vendored copy).

Validated in the torch-free container (healthz engines now emotion/features/
samples/grade/separate; a sine tone grades D with a correct seam-click flag) +
43/43 tests. Note: this is the CPU-doable slice of #31's /loops /grade /correlate
— the finder (/loops) and corpus correlation (/correlate) remain.