fix(validate): make --fix non-destructive and close geometry detection gaps

[LeadcodeDev]

Part of #102 — wave 1.

Mini-spec

Goal: make the geometry validator trustworthy — a non-destructive --fix, no false negatives on parent overflow, no blind spot on transform/camera, and honest animated sampling.

Invariants

• --fix never writes a property absent from CssStyle. A file it rewrites must re-validate at least as clean as before, and must never lose a component.
• A node clipped by an ancestor whose overflow is not visible is never reported as a viewport overflow.
• Content larger than its own content box is reported even when it stays inside the viewport.
• A violation path resolves to the same JSON node the validator measured, even when a sibling failed to deserialize.

Out of scope: schema changes (L4), font loading (L4), component-internal measurement (L5), the legibility floor (wave 2).

Acceptance

• C1 — on an unwrappable_text_overflow, --fix removes white-space: nowrap instead of adding wrap: true; the remediation hints stop naming wrap and font_size.
• H3 — with a deliberately invalid child at index 0, --fix patches the correct sibling.
• H4 — a shape fully inside an overflow: hidden card validates clean; a text wider than its overflow: visible card is reported.
• H7 — 420px type inside a full-frame overflow: hidden plane validates clean.
• H6 — --strict-anim no longer discards t=0 through the opacity guard, reuses the renderer's effective_effects / resolve_props_for_effects rather than a divergent fork, and samples proportionally to scene duration.
• H5 (partial) — static css.transform and scene.camera are folded into the bbox computation.
• Regression tests exist for each of the above (the module currently has 5 tests and none cover apply_fixes or --strict-anim).

Files: crates/rustmotion-cli/src/commands/geometry.rs, crates/rustmotion-cli/src/commands/validate.rs

Verify

rtk cargo test -p rustmotion-cli
rtk cargo build --release
for f in examples/*.json; do ./target/release/rustmotion validate -f "$f"; done

[LeadcodeDev]

Merged into main via #108 (squash 38596ca).