refactor(types,web): migrate usc_title to string for alphanumeric titles and appendices

[williamzujkowski]

Description

Migrates usc_title to support string values throughout the transformer frontmatter, Astro content collection schema, routing, and UI presentation. This ensures titles with letter suffixes (such as Title 18 Appendix 18a, 5a, 26a, etc.) are preserved without numeric truncation or collision.

Key Changes

• Frontmatter Validation: Updated FrontmatterSchema in packages/transformer/src/markdown-generator.ts to accept z.union([z.string().min(1), z.number().int().positive()]).transform(String).
• YAML Serialization: generateFrontmatter outputs numbers for pure numeric titles and quoted strings for alphanumeric titles.
• Section Generation: generateMarkdownForSection preserves titleNum as a trimmed string instead of forcing parseInt.
• Astro Content Schema: Updated apps/web/src/content.config.ts to accept string | number transformed to string.
• Navigation & Routing: Fixed browse/[title].astro and statute/[...slug].astro to support string keys.
• Title Dictionary: Added Title appendix entries (5a, 11a, 18a, 26a, 28a, 50a) to TITLE_NAMES in apps/web/src/data/title-names.ts.
• Natural Collation: Updated title list sorting in browse/index.astro to use localeCompare with numeric collation.

Closes #278