Motion blur: pointer paths, in-flow ghost placement, velocity-oriented smear, smooth whip, camera motion blur #427

Context

Follow-up of #359, #360, #361 and #362. A user building a one-take product film called camera and container motion blur one of their three main blockers.

To do

• pointer moved by path is never ghosted (motion_blur and trail ghosts take flex slots again, drop a node's children, and ignore pointer paths #359): a pointer with path and motion_blur renders pixel-identical to the same pointer without it; the same pointer moved by translate_x/y keyframes trails correctly.

  {"version":"1.0","video":{"width":1920,"height":1080,"fps":30,"background":"#FFFFFF"},"scenes":[{"duration":1.0,"children":[{"type":"pointer","position":"absolute","x":0,"y":0,"size":220,"tone":"dark","path":[{"time":0.0,"x":1900,"y":1000},{"time":0.6,"x":900,"y":500}],"path_easing":"linear","style":{"animation":[{"name":"motion_blur","samples":12,"shutter":3.0}]}}]}]}

• Ghosts of in-flow nodes are misplaced (motion_blur and trail ghosts take flex slots again, drop a node's children, and ignore pointer paths #359): they land where the parent's justify/align would put an inset-less absolute child, not on the node's own box. A node that does not move shows a ghost elsewhere (a band in the gap of a column; in a row "Hello" + "World" with motion_blur on "World", a ghost "World" is painted over "Hello").
• mode: "smear" is axis-aligned (A directional blur: blur_x/blur_y, a smear mode for motion_blur, and per-unit blur axis in char_* presets #360): a 45° move gives an isotropic blob instead of a diagonal streak. Orient the streak along the velocity.
• radius_x / radius_y (snake_case) validate and are silently ignored; only radius-x / radius-y work (A directional blur: blur_x/blur_y, a smear mode for motion_blur, and per-unit blur axis in char_* presets #360). Accept both or reject the snake_case form.
• whip streak is stepped (A whip transition: a slide with a directional streak along its axis #361): about ten discrete copies are visible mid-transition. Make it continuous.
• Camera motion blur (Camera: 3D rotation with a shared perspective, and camera motion blur #362, dropped for cost): a real user needs it. Implement it opt-in, e.g. camera.motion_blur: { "samples": 8, "shutter": 0.5 }, accumulating sub-frame camera positions only on frames where the camera moves, with the cost documented.

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.