Path drawing: draw_start on finished svg strokes, arrow/connector, draw_offset, svg d morph, dot at progress 0 #431

Context

Follow-up of #376 and #377.

To do

• draw_start is ignored on svg once draw_progress reached 1 or is not animated: the paint_resvg branch (svg.rs, around line 514) ignores it. A stroke drawn over 0–0.5 s with draw_start 0→1 over 0.9–1.4 s is never erased. This contradicts the rule doc, which says draw_start alone erases a finished stroke.
• draw_start keyframes validate and do nothing on arrow, connector and shape.
• draw_offset (marching the drawn window along the path) is rejected as unknown.
• Keyframes on d with target inside an inline svg are rejected ("unknown field target"); only shape supports path morph.
• Path morph between paths of different command structure passes validate, prints a render-time warning and holds the first shape: resample, or raise a named validation error.
• arrow and connector paint an 8×8 dot at draw_progress 0 (fixed for line in draw_progress: a dot at progress 0 on line, and an unscaled stroke without caps while an svg draws #376, not for these two).

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.