Review Section 5.1.2. Canonicalized Host Names

[colincasey]

Summary

• Restructures canonicalDomain.ts to map step-by-step to RFC 6265 Section 5.1.2
• Extracts canonicalizedHostName.ts containing the pure RFC algorithm (steps 1-3) separate from library-specific pre-processing
• Introduces labelToASCII helper for per-label IDNA conversion via the URL constructor's built-in UTS#46
• Replaces flat test block with 8 grouped describe blocks covering NR-LDH labels, U-label conversion, A-label passthrough, IPv6, dot handling, non-NR-LDH ASCII, and null/empty inputs
• Adds skipped test stubs for draft bis-22 validation (fake A-label rejection, invalid label type rejection)
• Comprehensive JSDoc documenting RFC 6265 algorithm, Section 6.3 IDNA guidance, URL constructor rationale, and bis-22 compatibility notes

Future work

Sub-issues created under #512 for follow-up refactoring discovered during this review:

• Deprecate canonicalDomain in favor of canonicalizedHostName #564 — Deprecate canonicalDomain in favor of canonicalizedHostName
• Add configuration to control RFC 6265 vs bis-22 behavior #565 — Add configuration to control RFC 6265 vs bis-22 behavior
• Move leading-dot stripping from canonicalDomain to domain attribute processing #566 — Move leading-dot stripping from canonicalDomain to domain attribute processing
• Move IPv6 handling from canonicalDomain to a separate utility #567 — Move IPv6 handling from canonicalDomain to a separate utility
• Implement draft bis-22 validation in canonicalizedHostName #568 — Implement draft bis-22 validation in canonicalizedHostName

Test plan

• All 29 canonicalDomain tests pass, 3 bis-22 stubs skipped
• Full test suite (791 tests) passes
• Lint clean on changed files
• Build succeeds, API surface unchanged (api-extractor clean)
• Type entrypoints verified (attw green)

Closes #512