feat(engine): spectral control backend (on models/backend)

[ryanontheinside]

Spectral control backend (activation steering), ported onto the backend seam

Merge order

Stacked: opened against ryanontheinside/feat/models/backend. Merge #224 -> #220 -> #229 before this.
Stack: main <- #224 <- #220 <- #229 (models/backend) <- this <- spectral demo PR. Sibling leaves off models/backend: mrt2 (#230), sa3 (#231) — this PR does not touch families.py, so it composes cleanly with both; the demo PR stacked on top carries the families.py heads-up.
Supersedes #162 (same feature, ported from the pre-seam ryanontheinside/feat/spectral/spectral-control; close that one in favor of this).

Engine-side activation steering: per-DiT-layer additive shifts on the post-block residual, gated per-row by denoise step, driven live from the streaming pipeline.

What's here (one commit, 39b83c3)

• acestep/steering/: new package — SteeringController (auto-axis vectors + manual catalog + LIFO slot registry + raw-knobs -> config translation), policy.py (pure axis/limits tables), catalog.py (vector enumeration/loading), hub.py (per-checkpoint HF bundle fetch/cache), types.py.
• acestep/engine/stream.py: set_steering() API; eager-path forward hooks on decoder.layers; TRT-path per-tick steering buffer fill; _current_step_per_row populated around the batched forwards so injection only fires at the targeted denoise step.
• acestep/engine/trt/export.py + build.py + runtime.py: decoder ONNX/engines gain a [B, num_layers, hidden_size] steering input (zeros = exact no-op). Engine filename prefix becomes spectral_decoder_*; pre-steering engines keep their old names and the streaming path degrades with a loud warning (knobs no-op until rebuilt).
• acestep/paths.py: steering-vector bundle paths per checkpoint; TRT profile table points at the spectral_decoder_* engines.
• Engine metadata schema bumped 1 -> 2 (decoder contract changed shape).

Port notes (vs #162)

• stream.py lands on the ModelAdapter seam: the per-row step mapping wraps _forward_pairs, which now dispatches through adapter.batched_forward (ACE) — TRT dispatch and the steering buffer fill stay pipeline-owned, so the injection semantics are unchanged.
• _ENGINE_METADATA_SCHEMA moved to its post-refactor home acestep/engine/trt/_engine_metadata.py; the 1 -> 2 bump lives there now.
• paths.py keeps both the seam's fixtures/user-uploads dirs and the steering helpers.

Validation

• 149/149 unit tests at this tip (full inherited guard suite).
• Golden harness: pending a free GPU window; will be run on the demo tip (which contains this commit) and reported there.