fix(codegen): keep a percentage line advance when the font size is bound to a variable

[forestream]

What

validate_line_metrics refused any TEXT whose lineHeight is PERCENT while its fontSize is bound to a variable:

Text node '...' cannot represent a size-dependent line advance without resolved
fontSize and lineHeight; variable sizes require mode-aware metrics.

The guard is right about the danger and wrong about the representation.

Right: a pixel advance derived from a resolved size is only true in the mode that resolved it. Emitting lineHeight="27px" for a size that a variable can change publishes a number that silently stops matching.

Wrong: the value was representable all along. A Figma PERCENT line height and a CSS percentage line-height mean the same thing — both scale with whatever font size applies. Leaving the ratio alone is exact under every mode, not approximate under one.

So line_height now keeps the ratio when the size is bound, and line_height_font_size is what decides that: a bound size is withheld from the conversion rather than being trusted as a constant.

Scope of the change

• Unbound sizes keep the pixel conversion. Every golden is unchanged — which is also the evidence that no existing screen relied on the refusal.
• The refusal now fires only when the percentage itself is missing, which is the case that genuinely carries no value to emit.
• The theme path is untouched. generate_devup_json still refuses a bound size, and variable_font_size_cannot_publish_a_fixed_percentage_advance still passes: a typography token records one pair of numbers for all of its uses and cannot carry a ratio the way an element can.

Evidence

A 1920-wide screen from a design system that binds every type size to a variable. Before: no code at all — the export failed at projection-and-write with DEVUP_CODEGEN_FAILED, and so did all three of its separately addressable siblings. After:

• 71 texts keep lineHeight="150%"
• 6 unbound texts keep their pixel advance (lineHeight="21px")
• 1,430 lines of tsx where there was previously none

Tests

integer_line_advance::variable_font_size_keeps_the_percentage_advance_instead_of_a_pixel_one — new; asserts the ratio is emitted and the pixel form is not. Fails before the change.

Gates

Gate	Result
cargo fmt --all -- --check	pass
cargo clippy --locked --workspace --all-targets --all-features -- -D warnings	pass, zero warnings
cargo test --workspace --release	pass except visual_compare::symlink_escape_refused_before_read, which fails identically on an unmodified tree (Windows symlink privilege)