Implement string override system

[Crauzer]

Implement string override collection, priority resolution, and application to overlay.

Requires completion of #84 (mod metadata schema) and #96 (string table format research).

See implementation plan in docs/overlay-implementation-plan.md Phase 3.

Acceptance Criteria:

• Can collect overrides from multiple mods
• Priority resolution works correctly
• Can parse and write string tables
• Integration test verifies behavior
• In-game testing confirms strings changed

Related: Epic #85