Cache brief category metadata lookups during builds

[plx]

Summary

Category metadata loading currently performs synchronous filesystem reads (existsSync + readFileSync) during card prop generation, potentially repeating reads per brief card.

Affected files:

• src/lib/category.ts
• src/lib/contentCardHelpers.ts
• src/pages/briefs/index.astro

Why This Matters

Repeated sync I/O is non-idiomatic for repeated lookup paths and can become a build-time bottleneck as content grows.

Proposed Strategy

• Add an in-memory cache (e.g., Map<string, BriefCategory>) in category utilities keyed by category slug/path.
• Ensure each category’s YAML is read and parsed at most once per build.
• Keep current fallback behavior (default category metadata when YAML missing/invalid).

Done Criteria / Validation

• Category lookup path is cached and no longer performs per-card sync reads.
• npm run build passes.
• npm run qa passes.
• Brief listing/category pages still display expected category prefix/description values.