Transitions: feathered wipes leak at fixed edges, clip-path blob, iris pill, warnings for ignored fields #432

Context

Follow-up of #364 and #383.

To do

• Feathered wipe_* leak at the fixed frame edges (Transition edges: a mask transition with any silhouette, an organic blob, and feathered or tinted wipes #383): with feather / band_color, the reveal rectangle is feathered and tinted along the three frame edges that should stay fixed. wipe_left, feather: 320, band_color: #FF8CC6, at t=0.8: pixel (640,3) is pink instead of the incoming scene; with feather: 120 and no band, frame edges are a 50% blend of both scenes. Only the moving edge must be feathered.
• clip-path: {kind: "blob"} does not exist (Transition edges: a mask transition with any silhouette, an organic blob, and feathered or tinted wipes #383 asked for it; neither the PR nor the docs mention it).
• mask error messages (Transition edges: a mask transition with any silhouette, an organic blob, and feathered or tinted wipes #383): passing shape instead of silhouette gives "invalid value: map, expected map with a single key"; a fractional origin (0.5) is silently read as 0.5 px.
• iris shape: "pill" renders a rectangle with ~5 px corners instead of a stadium (iris accepts origin and ignores it; it also needs a shape, a fill colour and a ring #364).
• No warning for fields a transition type ignores (iris accepts origin and ignores it; it also needs a shape, a fill colour and a ring #364): {"type":"fade","duration":0.6,"origin":{"x":10,"y":10},"ring":{"color":"#fff","width":4},"shape":"pill"} validates silently.

Definition of done

• A test that fails on current main is written first. For anything visual it is a render test (scenario → frame → pixel assertions); tests that build AnimatedProperties by hand are not enough — that is exactly how the regressions listed here slipped through.
• Each item is verified end to end: scenario JSON, rustmotion validate, render, image inspected.
• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace pass.
• Vocabulary changes update crates/rustmotion/skills/SKILL.md, the relevant rules/ file and the exported schema. A field the validator accepts must have an effect, otherwise the validator rejects it or warns.
• No comments in Rust code (repository rule). Commits follow <type>: <verb> <message>. No AI attribution in commits, PRs or issue comments.