Create helper functions to support plan_rewrites() extraction: - build_local_block_map(): Build block ID mapping for a function - sync_spans(): Synchronize instruction spans after rewriting These pure functions will be used by both the current monolithic merge_and_rewrite() and the new plan_rewrites() function. Progress: Step 1/4 (helpers) complete 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>