Playbook: add a stitch type

A stitch type touches several registries, but each touch is small. Follow the steps in order; the gates catch anything you miss.

1. Design page first. Add or extend the section in docs/src/design/algorithms/ with: purpose, parameters (registry keys; Ink/Stitch names where they exist), the algorithm in your own words, properties, diagnostics, references. Never copy code (ADR-0012).
2. Requirements. Add REQ-… entries to conformance/requirements.toml with status = "planned" and the milestone.
3. Cases. Add cases under conformance/cases/<area>/: typical shapes, the degenerate corpus entries that matter, and one case per diagnostic. They fail for now — that is the point.
4. Module. Create crates/stitchcraft-engine/src/generators/<name>/ with mod.rs (the Generator impl), params.rs (the params! block, doc comments as help text) and tests.rs. Keep files under 800 lines; split by concern (sampling, routing, compensation).
5. Register the generator in the generator table and its params in the registry list (one line each).
6. Make the cases pass. Run cargo xtask conformance --filter <area>; switch the requirements to active.
7. Docs. cargo xtask docs regenerates the parameter pages; write the user-facing page in docs/src/user/ (what it is for, when to use it, two or three declared shots); run cargo xtask shots.
8. Gate. cargo xtask ci, then open the PR with the roadmap step id.
9. Sew-out. If the type is new to the machine, list it for the next machine checkpoint (TS-11).