feat: scan embedded framework files with custom language descriptors

[FanaticPythoner]

Add descriptor-backed language configuration so framework components and custom
compound files scan nested comment syntaxes while preserving base document
matches, offsets, and workspace scan parity.

Manifest and configuration:

• add Languages settings for custom comment patterns and embedded documents
• expose current and legacy namespaces with schema and localized descriptions
• snapshot language settings and rebuild scanner state after language changes

Comment pattern resolution:

• move extension aliases into shared data for JS, TS, JSONC, JSON5, Dart, Stylus,
  Vue, JSX, TSX, and module variants
• add a catalog that resolves ids, aliases, extensions, filenames, compact
  tokens, and MIME subtypes against comment-pattern metadata
• compile custom patterns with single-line, multiline, regex-start, glob, and
  comments-only support
• fail invalid custom language definitions with typed configuration errors

Embedded documents:

• add descriptor data for Vue, Svelte, and Astro components
• add parser registry and html-like region parser for elements, attributes,
  raw-text blocks, frontmatter fences, wrapped regions, and unresolved masks
• merge custom embedded descriptors ahead of defaults through the settings cache

Scanning:

• mask embedded ranges during base scans and scan nested regions with their own
  comment patterns
• shift nested result positions, offsets, and capture ranges back to the parent
  document
• resolve documents by filename, extension, glob, language id, and pattern name
• keep streaming scans alive across open embedded regions
• reuse snapshot resource config for raw ripgrep normalization and avoid exact
  regex execution when captures are not needed

Evidence and tooling:

• add custom language and issue 19 embedded-document benchmark harnesses
• extend perf scenario stubs with language resolver hooks
• allow regex equivalence evidence loading from staged JSON modules
• update registry equivalence and embedded-document benchmark artifacts

Coverage:

• add custom language, embedded descriptor, parser registry, Vue, Svelte, Astro,
  workspace includeGlob, streaming, resolver cache, parity, settings, and alias
  tests
• extend parity fixtures and tolerated deviation rules for comment-aware framework
  aliases
• cover glob-magic registry behavior and manifest setting counts

Fixes #19