Cache normalized turns/calls to ~/.cache/codeburn/session-cache.json so
the CLI skips re-parsing unchanged JSONL files on subsequent runs.
File reconciliation uses dev+ino+mtime+size fingerprinting; cost,
classification, and summaries are recomputed at query time. Atomic
writes via temp+fsync+rename, deep structural validation on load,
per-provider env fingerprinting, and best-effort save so cache failures
never break the CLI. ~6x speedup on warm cache.
