Preserve SVG fragments in CSS image URLs

[nexiumbiz-debug]

Summary

• preserve SVG fragment identifiers such as #svgView(...) when CSS image URLs are embedded as data URLs
• fetch/cache the actual SVG resource without the fragment, then append the fragment back onto the generated data URL
• add a regression test for CSS url("...svg#svgView(...)") replacement

Fixes #142.

Testing

• corepack pnpm exec vitest run test/nodejs.test.ts --pool=forks --maxWorkers=1 --minWorkers=1 --no-file-parallelism
• corepack pnpm exec eslint src/css-url.ts test/nodejs.test.ts
• corepack pnpm exec tsc --noEmit
• git diff --check

Note: the default Vitest worker-pool run crashed under local Node 24 before collecting tests, so I reran the focused test file with one worker and it passed.