fix(streamdown-rn): optimize splitter and harden URL sanitization

[ronak-guliani]

Summary

Fixes two issues in streamdown-rn:

1. Performance/DoS risk in splitter processing path.
2. URL policy bypass for protocol-relative URLs and URL-like props.

What changed

• Refactored splitter from per-character full rescans to incremental appended-content processing.
• Added explicit close-index detection for code blocks and component blocks, with remainder processing in the same update.
• Hardened URL sanitization:
  • Blocks protocol-relative (//...) and UNC-style (...) URLs.
  • Sanitizes URL-like prop keys even when values don’t match protocol regex.
• Added regression tests for splitter boundary/chunk-size behavior and URL sanitization edge cases.

Verification

• bun test (repo root): pass
• bun run type-check (repo root): pass
• bun run build (repo root): pass

Performance note

Quick local benchmark (production mode) improved from ~4000: 260ms / 8000: 953ms to ~4000: 1.82ms / 8000: 1.44ms for the audited splitter scenario.