M13.35: Roto Brush trained model — swappable segment module, MobileSAM in pure Rust

- New L1 crate effectcraft-segment: the MaskModel interface (prompts → foreground probability),
  a registry of open-licensed models (licence must be MIT/Apache/BSD/…; URL, size, SHA-256),
  a PyTorch checkpoint reader (zip + pickle subset), dependency-free SHA-256, and MobileSAM
  (TinyViT-5M + SAM prompt encoder / two-way transformer / mask decoder) on rayon and ndarray's
  safe GEMM. Loads the official mobile_sam.pt as published; checked against candle
  (embedding 0.3% RMS, 99.9% equal mask pixels); ~0.7 s/frame encoder at 32 threads, cached
  per frame, ~0.1 s decoder.
- track::roto: segment_with / propagate_with prompt a model with the strokes or the flow-warped
  matte (box, core points, mask); its probability is a strong, unblurred prior for the same
  graph cut (strokes stay hard constraints); a failing model or one disagreeing with the flow
  (IoU < 0.5) is ignored. Prior gets a weight.
- effects::roto: Version 2.0 / 3.0 use the model chosen in Settings (set_model / model_for);
  the model id is part of the chain seed.
- engine: roto_models (roto.models, roto.model.select / install / download / remove), weights in
  the models folder (host::config_dir()/models, shared by the app, CLI and MCP), background
  loading, downloads through the system curl, verified; Settings ▸ Roto Brush page (prefs
  roto.model). host::config_dir moved from the desktop app.
- Tests: segment (kernels vs direct sums, checkpoint reader, SHA-256 vectors, registry), track
  (fake model: prior shapes the cut, failing / disagreeing models ignored; real weights with
  EFFECTCRAFT_MOBILESAM: base IoU 0.989, propagated ≥ 0.980 vs classic 0.973), engine (registry,
  verification, selection; real install + load), MCP.
- Docs: README, ROADMAP, parity (MSK-4 done: 90/2/0), preferences, agents, architecture,
  crates/segment/README.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

xtask/src/main.rs

@@ -32,6 +32,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("keyframe", 1),
     ("path", 1),
     ("psd", 1),
+    ("segment", 1),
     ("project", 2),
     ("text", 2),
     ("effects", 2),