fix: emit valid CSS for text shadows

[Iams4kura]

A Figma TEXT node with a drop shadow currently emits 2px 3px 4px 0px rgba(0, 0, 0, 0.5) as textShadow. Browsers reject the fourth length and compute text-shadow: none, so even an ordinary text drop shadow disappears when the returned CSS is used.

Text shadows now use the supported three-length form, 2px 3px 4px rgba(0, 0, 0, 0.5). Inner-shadow entries are omitted for TEXT nodes because inset invalidates the entire list, including otherwise valid drop shadows. Non-TEXT box shadows retain their spread and inset values; blur output is unchanged.

CSS text-shadow cannot represent Figma spread or inner shadows; this fix does not attempt to emulate either. The grammar is defined in CSS Text Decoration §4.

Validation:

• Same regression tests on base c083d65c7e002923e7cb98f4e3bdafb105e90f6d: 5 failures before, all 9 effects tests pass after. Covers absent/zero/nonzero spread, mixed and inner-only shadows, hidden effects, multiple shadows, blur preservation, and box-shadow compatibility.
• pnpm test: 256 passed, 1 skipped. The skipped test is the repository's opt-in live Figma integration test; no Figma account or design file was used.
• pnpm type-check, pnpm lint, pnpm build, pnpm exec prettier --check "src/**/*.ts", and node scripts/scan-hidden-chars.mjs pass (Node 26.0.0, pnpm 10.10.0).
• Chrome 147: CSS.supports("text-shadow", before) is false and the computed value is none; the actual transformed output after this fix is supported and renders a shadow. The non-TEXT computed box shadow is identical before and after.

Only the effects transformer and its tests change. No dependencies or generated files change.