ENH: tree-shake unused AMP CSS custom properties + trim @font-face to woff2

[hotinglok]

Why

Builds on #14229. Further reduces the size of the inline <style amp-custom> on AMP article pages to keep well under AMP's 75KB CSS limit.

The inline AMP CSS is assembled from Emotion critical CSS + the concatenated SCSS-module chunks + the active service theme (:root custom-property blocks + @font-face). Two AMP-only passes are added to optimiseAmpCss in _document.page.tsx.

What

• treeshakeCssCustomProperties — removes CSS custom properties from :root {} blocks that are never referenced via var() in the final CSS. Service themes declare their full palette and font-variant token sets on :root, but a given page references only a fraction. Includes transitive-closure handling so --a: var(--b) keeps --b when --a is used.
• trimFontFaceSourcesToWoff2 — trims each @font-face src list to its woff2 source (every AMP-capable browser supports woff2), preserving local() and leaving a face untouched if it has no woff2 source.

Both run only on the AMP article path (isAmp && pageType === 'article'), so canonical/Lite are unaffected.

Measured impact

Against the real news theme (latin, 4 font faces, ~6.1KB theme CSS) with the actual transforms:

Transform	Saving
tree-shake custom properties	~4.7–5.0KB
woff2 trim	~0.4KB

Tree-shake is the dominant lever: components consume theme colours via compile-time SCSS $ variables (literal hex), so the theme's --brand-*/palette custom properties have effectively no var() consumers in the rendered CSS and are safely removed.

Safety

• Verified there are zero inline style="" references to var(--…) in the codebase — stylesheets are the only consumer — and AMP runs no custom JS, so the final concatenated CSS string is the single source of truth for what is used.
• Tree-shake only drops properties unreferenced within the very string it minifies, so it cannot break a live reference.

Tests

• treeshakeCssCustomProperties/index.test.ts — removal, transitive keep, empty-block removal, non-custom-property preservation, multi-:root/dedup usage.
• trimFontFaceSourcesToWoff2/index.test.ts — woff fallback removal, multiple legacy formats, local() preservation, no-woff2 untouched, multi-face, no-@font-face untouched.

All 11 unit tests pass; new files pass Prettier.

Notes

• A dedupe pass was prototyped and dropped — measured ~0 bytes because the theme's :root/@font-face are emitted once and component rules are uniquely scoped, so there are no byte-identical top-level duplicates in this architecture.