chore: phase B (jsonl batch insert) complete

ACs implemented:
- batch-jsonl-basic (test, B3 → 25f42b7)
- batch-missing-key-rejected (test, B4 → b3e6983)
- batch-duplicate-key-in-stream-rejected (test, B5 → b958d8c)
- batch-collision-with-existing-record + git-based rollback (B6 → 89edc77)
- batch-empty-stream-succeeds (test, B2 → 1937b29)

Deferred to code-inspection-only verification (no runtime fixture in MVP):
- batch-view-materialization-once
- batch-post-commit-view-failure

Architectural finding addressed: pkg/dalgo2fsingitdb's
RunReadwriteTransaction is a stub with no rollback. Batch atomicity is
now provided by an orchestrator-level rollback in insert_batch.go that
restores tracked files via 'git checkout HEAD --' and removes untracked
files via os.Remove (falls back to remove-only when not a git working
tree). The dalgo backend itself is untouched.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: inGitDB Dev