Add local batched Git blob reader

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Computational Efficiency Implementation Plan
 
-**Status:** Stages 1–3 complete; Stages 4–11 pending.
+**Status:** Stages 1–4 complete; Stages 5–11 pending.
 
 ## Purpose And Baseline
 
@@ -236,6 +236,24 @@ by deterministic cleanup. Update the process inventory description when needed.
 **Owners:** A focused reader file and tests under `internal/git`, reusing
 [source.go](../../internal/git/source.go).
 
+**Completion:** Complete. `BlobReader` lazily starts one local-only
+`git cat-file --batch` process, serializes request/response exchanges, and
+returns caller-owned body bytes after validating the object ID, blob type,
+representable size, and framing. Missing objects produce contextual errors
+without invalidating a healthy session. Broken exchanges and cancellation close
+the pipes and reap the child, preserving the original failure, Git stderr, and
+process causes. Existing standalone blob and committed-file readers remain in
+place. Tests cover repeated loose and packed reads, empty and binary bodies,
+missing objects, partial clones, malformed/truncated responses, cancellation,
+and child failures.
+
+The process-reuse benchmark used one 9-byte blob. A new reader per read took
+1.98–2.24 ms and allocated 37.9 KB in 190 allocations per operation; a shared
+session took 57–82 μs and allocated 425 B in 8 allocations. The fresh-session
+measurement includes process startup and clean close per read. The shared-session
+timer includes lazy startup on its first read; final cleanup is outside the
+timer. This is a fixture-specific measurement, not a subprocess-count contract.
+
 1. Add an internal `BlobReader` that stores a repository directory and parent
    context. Create its process lazily on the first content request through
    `process.StartPiped` with `git cat-file --batch`.