fix(painter-dom): set default page text color

[xy200303]

Summary

• Give DomPainter pages an explicit default text color of var(--sd-layout-page-color, #000).
• Allow consumers to override the page text color through pageStyles.color.
• Add tests covering the default and override behavior so list markers and other content without explicit run color do not inherit a hostile host-app color.

Verification

• pnpm test:all -- --root ./packages/layout-engine/painters/dom src/styles.test.ts
• git diff --check
• rg "@superdoc/(super-editor|style-engine|layout-bridge|layout-resolved)" packages/layout-engine/painters/dom/src -g '!*.test.ts' -g '!_test-utils.ts'
• pnpm test:all -- --root ./packages/layout-engine/painters/dom src/paragraph/list-marker.test.ts src/renderer-marker-suffix.test.ts src/renderer-marker-textwidth.test.ts (fails during collection: current workspace cannot resolve @superdoc/font-system)
• pnpm test:all -- --root ./packages/layout-engine/painters/dom src/index.test.ts -t "word-layout markers|resolved list" (fails during collection: current workspace cannot resolve @superdoc/font-system)

Closes #3456

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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