Reduce decorative SVG preserve-dom fallbacks

[chubes4]

Summary

• Convert explicitly decorative inline SVGs into native image blocks in preserve-dom reconstruction.
• Keep ambiguous unlabeled SVGs as HTML fallback islands.

Validation

• ./node_modules/.bin/vitest run src/theme/preserve-dom/tests/strategy.test.ts
• ./node_modules/.bin/tsc --noEmit

Residual Risk

• corepack pnpm scripts are blocked locally by pnpm's esbuild build-approval policy; direct local binaries passed after dependency materialization.

AI assistance

• AI assistance: Yes
• Tool(s): GPT 5.5 via OpenCode, plus Lab OpenCode subagent
• Used for: Drafted the focused patch, rebased it locally, and ran validation.