fix: Decode percent-encoded image data URLs

[GtechGovind]

Fixes #2126.

I added percent-decoding for image data-URL payloads so encoded SVGs and binary images load correctly. Decoding works on bytes rather than using decodeURIComponent, which rejects valid non-UTF-8 image bytes and literal percentages in SVGs.

• Decode valid %xx escapes once, preserving raw UTF-8, plus signs, and malformed escape sequences.
• Decode percent escapes before base64, as specified by the data-URL processing algorithm.
• Keep the existing fast path for payloads without percent signs. File, Buffer, HTTP, and object-URL loading are unchanged, as are source getters and callback behavior.
• Add 15 decoder tests and five image integration tests, including rendered pixel comparisons. Update the changelog.

Validation on macOS Apple Silicon:

• zig build -j4 with Zig 0.16.0: passed.
• npm test -- --reporter dot on Node 24.19.0: 325 passed, six existing pending tests.
• node node_modules/mocha/bin/mocha.js --reporter dot 'test/*.test.js' on Node 26.0.0: 325 passed, six existing pending tests.
• npm run lint, npm run tsd, and targeted Standard checks for the new decoder/test files: passed.
• The three encoded PNG/base64/SVG regression cases failed with Unsupported image type before the implementation and pass afterward.
• An additional 144 deterministic payload combinations matched Node's fetch(dataURL) byte output, covering UTF-8, binary bytes, plus signs, literal percentages, and partial/invalid escapes.

Linux/Windows and the generated WPT suite were not run locally. This change does not attempt to overhaul existing data-URL metadata parsing or base64 validation.

• Have you updated CHANGELOG.md?