fix(text): share decoration geometry between Canvas and SDF renderers

[wouterlucas]

Part 4 of 4. Base is #909 — review that first; this diff is isolated against it.

Problem

Underline and strikethrough offsets were defined independently per backend and disagreed, so identical markup rendered at different heights depending on which renderer was active:

	Canvas	SDF
Underline gap	fontSize * 0.08 (CanvasTextRenderer.ts:238)	fontSize * 0.1 (SdfTextRenderer.ts:407)
Strikethrough	ascender - ascender * 0.4 → 60%	base * 0.75 → 75%

Approach

Offsets now come from RichTextMetrics.

Each backend keeps its own baseline source, since those differ for good reason — Canvas uses normalized font metrics, SDF uses the BMFont base value, which is more accurate for its atlases and is already documented as such in a comment there. Only the offsets from the baseline are unified.

The Canvas values were adopted, so SDF underline and strikethrough move slightly.

⚠️ Requires snapshot regeneration

Visual regression snapshots covering SDF underline or strikethrough will need regenerating. I was unable to run pnpm test:visual locally (needs Docker/browser), so this has not been verified visually — please confirm before undrafting.

On the strikethrough ratio specifically: a true strikethrough is centred on the x-height, but neither backend has x-height available, so it is approximated as a fraction of the top-to-baseline distance. 60% (Canvas) sits slightly high, 75% (SDF) slightly low relative to a computed ideal of roughly 69%. I adopted the Canvas value rather than inventing a third number, but if you would rather split the difference, STRIKE_BASELINE_RATIO is a single constant.

Also includes

examples/tests/text-rich-spacing.ts — renders the same styled markup on both renderers with a guide box tracking each node reported width and a rule marking maxWidth. Covers the reported defect, italic shear overhang, adjacent styled runs, trailing styled runs, wrapping driven by styled width, mid-word splits, and centred alignment. Step through with →.

Testing

tsc --noEmit clean, 336 tests pass, eslint 0 errors. Adds 5 tests covering sub-pixel stroke clamping, underline gap floor and scaling, and strikethrough placement.