fix(css): resolve em against the element font-size

[LeadcodeDev]

Severity Low, category correctness. Location: crates/rustmotion-core/src/css/taffy_bridge.rs:292

Impact

build() (layout_pass.rs:122-154) hands the same ConversionContext to to_taffy_style for every node in the tree and never re-derives it from the node's own (cascaded) font-size, and the only production builder keeps ..LengthContext::default() → font_size: 16.0 (scene.rs:29-37). So padding: "1em", gap: "0.5em", width: "10em", top: "2em" all resolve against 16px regardless of the element's actual font size: on a 48px-font card, padding: "1em" reserves 16px instead of 48px. Unlike the context-free .px() accessors — which were deliberately made to warn loudly for exactly this class of drop (units.rs:240-256) — this path is silent, so the wrong geometry reaches both the renderer and the validator with no signal.

Fix

Derive a per-node LengthContext while building the taffy tree: resolve the node's own font-size (post-cascade) to px and pass it as ctx.length.font_size to its own to_taffy_style call and to its children's. Until that exists, emit the same one-shot warning order already uses (taffy_bridge.rs:142-151) whenever an em reaches this bridge.

Evidence the audit read

ParsedLength::Em(em) => tf::LengthPercentage::length(em * ctx.length.font_size),
ParsedLength::Rem(r) => tf::LengthPercentage::length(r * ctx.length.root_font_size),

Based directly on the chantier branch.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-26).