Two new CPU-native, torch-free, cached endpoints on the audio sub-API — the
"interesting value points" for hexa beyond emotion, each the same shape/pattern
as /analyze/emotion (upload → content-address → cache → compute).

- POST /features  → the ~35-dim audio feature stack (spectral moments, MFCCs,
  chroma/key, envelope/attack-decay, + rhythm/tempo). scope `features`.
- POST /analyze/samples → per-sample EDA + role (percs|bass|melodic|tops|atmos)
  decided by the MEASURED spectrum (centroid + band energy), never the name; an
  optional ?name= only disambiguates breaks/drums (feedback_mastering_eda). scope `samples`.

Engine: engines/feats.py is SELF-CONTAINED (vendored DSP), like ears_light —
the deployed container holds only armada/api/, not armada/tide-table/, so it can
NOT import sample_features/audio_lens at runtime. It mirrors their algorithms and
fixes the librosa-0.11 tempo bug (feature.tempo, not the removed
feature.rhythm.tempo that silently dropped tempo in the tide-table original).

Wiring: a shared _cached() helper now backs all three analyze routes (DRY);
healthz advertises the engines map; both routes are scope-gated via the gateway
X-Scopes. Validated in the torch-free container: /features 200 (41 features,
tempo 107.7, key=Amaj), /analyze/samples role-by-measurement, 403 scope gate,
X-Douanier-Cache hit on repeat. 40/40 tests (added test_feats.py + endpoint cases).