Adds the apply hook for ActAdd-style activation steering on the Llama
backbone. Inspired by EmoSteer-TTS (arXiv 2508.03543), but adapted: the
paper is flow-matching-specific (DiT layers, 32 CFM steps, per-token
attribution search via mel synthesis), none of which apply to CSM's
autoregressive Llama-over-Mimi-tokens. What's portable is the
underlying difference-in-means construction with residual-stream
addition — the standard ActAdd / contrastive-steering pattern.
What lands:
- src/steering.rs: LayerSteering type, per-layer (1, embed_dim) tensors,
global scale, safetensors load with keys `layer_<i>_steering`. Three
unit tests covering empty/no-op, dimension validation, and apply math.
- src/csm_fork.rs LlamaModel: optional `steering: Option<LayerSteering>`
field, applied after every layer's forward inside the for-loop. Adds
~3 LOC to the hot path; gated by the Option so unsteered generation
has zero cost beyond a None check.
- src/csm_fork.rs Model::set_backbone_steering: installs steering only
on the conditional backbone (cfg_backbone is intentionally left
un-steered so CFG correctly subtracts an unsteered baseline).
- src/generator.rs Generator::set_steering: errors on quantized
backend (only FP supported for now).
- examples/generate.rs: --steering-vec / --steering-scale flags.
- examples/steering_random.rs: smoke helper that writes random Gaussian
vectors so the apply path can be exercised end-to-end before the
real corpus extractor lands. Box-Muller via seeded rand to avoid an
extra rand_distr dep.
Smoke test (16-layer random Gaussian, stddev=0.05, scale=0.5):
- baseline (no steering, same seed/text): 3.04 s @ RMS -19.5 dB
- steered (random vectors): 1.84 s @ RMS -16.2 dB,
EOT triggered earlier
Output clearly differs — pathway is wired correctly. Random vectors
aren't musically meaningful; that's Phase B.
Phase B (next session): corpus extractor that runs forward passes over
emotion-labeled audio (we already have audio_to_manifest emitting
emotion_tag rows), captures per-layer post-residual activations, and
computes the difference-in-means between emotion_X and neutral pools.
Then A/B with quality_eval.
Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
