fix(rebrand): preserve brand casing across forks

[TortoiseWolfe]

Closes #933

Summary

• replace source-brand content case-insensitively while preserving display, slug, identifier, title-case, uppercase, and arbitrary mixed-case projections
• rename complete tracked paths through an atomic collision-checked plan, with stale-index and symlink safety preflights
• independently verify eligible content and paths have no unmarked old-brand survivors before committing the new identity state
• add fork-stable unit, shell, mutation, retry, collision, and downstream rebrand regressions plus updated forking contracts

Verification

• pnpm run test:scripts — 602/602
• bash tests/rebrand/test-rebrand.sh — 63/63
• clean destructive fork to geo LARP — rebrand succeeded; 602/602 script tests, ESLint, TypeScript, and fork smoke passed
• enforced pre-push quick CI — secret scan, ESLint, TypeScript, breakpoints, production build, 111-chunk parse check, and first-load budget passed