Validator and vocabulary consistency: arrow/connector bounds, overflow hints, gradient stops, duplicate warnings #437

To do

• arrow / connector bounds (Geometry validator: a line's bounding box ignores x1/y1 #369): a fixed padding (half stroke + 16 + arrow_size) applies on every side even without arrowheads → false positives within ~30 px of an edge. curve and routing: "curved" are ignored → a curve leaving the frame passes validation (an arrow from (1700,100) to (200,100) with "curve":0.5 has its ink clipped at y=0 and validates).
• Overflow hint names the wrong edge for left/top overflows ("current right edge is 402" for a left edge at −102); the generic div hint has the same fault.
• Gradient stops vocabulary (gradient_text: the default angle stretches the ramp far beyond the glyphs, and there is no stops field #368): gradient_text uses position, backgrounds use offset (rejected on gradient_text): unify or accept both. shape.fill linear keeps the old angle convention (0 = left→right), inconsistent with gradient_text and background.
• The "overlap longer than transition" warning is printed twice at validate and three times at render (Transitions silently dropped: on a view's first scene, and on every scene of a view that contains one overlap #371).
• validate prints durations with one decimal ("3.0s" for 2.95 s) (Duration accounting: info ignores transitions and at, and per-scene rounding adds a frame #372).

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.