Refactor cache_rebuild_internal() - Split 67-line monster

[flyingrobots]

Part of SRP Refactoring (Priority 1: Core Infrastructure)

Current Problem

cache_rebuild_internal() in src/cache/builder.c is a 67-line function doing everything - violates SRP.

Refactoring Plan

Split into focused functions:

• prepare_cache_rebuild() - Setup temp directory and context
• collect_all_edges() - Gather edges from journal
• build_edge_maps() - Create forward/reverse mappings
• persist_cache_data() - Write bitmaps and trees
• finalize_cache_commit() - Create commit and update refs

Acceptance Criteria

• Original function replaced with orchestrator calling new functions
• Each new function has single responsibility
• Each function ≤ 20 lines
• Unit tests for each new function
• Integration tests still pass
• Performance not degraded

Technical Notes

• Use dependency injection for testability
• Extract pure functions where possible
• Document each function's responsibility

Related to SRP_REFACTORING_PLAN.md Priority 1.1