fix(geometry): resolve transform lengths per axis and per font-size

[LeadcodeDev]

Severity Medium, category correctness. Location: crates/rustmotion/src/cli/commands/geometry.rs:477

Impact

apply_transform_chain resolves translate/translateX/translateY lengths through this context. The paint pass resolves the very same functions through LengthContext { ..., parent_size: box_layout.width.max(height), font_size: node.css.font_size_px_or(16.0), ... } and then splits it into per-axis length_ctx_x/length_ctx_y (crates/rustmotion-core/src/engine/paint_pass.rs:229-250, used at :953-962). Two divergences: (a) em — a font-size: 96px text with transform: "translateX(-10em)" is moved -960px by the renderer but only -160px by the validator, so a component at x=200 that renders at x=-760 (fully off-frame) validates clean; (b) percentage translate — the validator uses max(w, h) on both axes, the exact mistake paint_pass.rs:236-242 calls out in a comment ("never max(width, height) on both axes"), which over-estimates on the short axis and produces false positives. Note the --strict-anim path funnels animated transforms through this same function (geometry.rs:1482), so it inherits both errors.

Fix

Build the context from the node's resolved font-size (css.font_size_px_or(16.0)) and resolve the x-component against bbox.w and the y-component against bbox.h, mirroring length_ctx_x/length_ctx_y in paint_pass.rs:243-250.

Evidence the audit read

let ctx = LengthContext {
    viewport_width: viewport.0,
    viewport_height: viewport.1,
    parent_size: bbox.w.max(bbox.h),
    font_size: 16.0,
    root_font_size: 16.0,
};

Stacked on fix/geometry-viewport-units, which carries the previous finding of this workstream. GitHub shows only this finding's diff; merge in order.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-15).