feat(p6-batch): residual-batch write/read driver + codec

Implements the client-only residual-batch feature end-to-end:
- src/shared/batch.{c,h}: self-contained single-file batch codec using the
  existing chunk_serialize/chunk_deserialize codec (byte-identical by
  construction).  Magic+format-version header (metadata mode is persisted into
  the header so a batch is self-describing across machines), length-prefixed
  chunk records, bounded reads that reject malformed/truncated/oversized
  records cleanly.
- src/client/client_send.c: write_batch_from_source (deterministic separate
  scan pass, loads every chunk's file images, emits header+records) and
  apply_batch_to_dest (local apply to a destination root via
  file_save_to_disk_full).  No wire change, no server involved.
- src/client/client_validation.c: --write-batch XOR --only-write-batch;
  --read-batch exclusive with both; --read-batch needs only a DEST,
  --only-write-batch only a SOURCE.
- src/client/client_cli.c: main() drives the three batch modes without
  connecting/transferring for read/only-write; --write-batch runs the live
  transfer (single-threaded so the config survives) then emits the batch.
- tests/test_batch.{c,h} (unit: byte-identical roundtrip with and without
  metadata; bad-magic/truncated/oversized rejection) + tests/integration/
  test_batch.py (only-write no-server, read-batch no-source roundtrip,
  --write-batch with a live transfer, conflict rejections).
- clang-format: realign PART-1 config.h comment block.

No PROTOCOL_VERSION bump, no config-frame field, no server flag.

src/shared/batch.h

@@ -0,0 +1,24 @@
+#ifndef BATCH_H
+#define BATCH_H
+#include "chunk.h"
+#include "config.h"
+
+/* Phase 6 residual-batch codec.  A residual batch is a self-contained
+ * single-file record of a whole source tree: a magic+format-version header
+ * followed by length-prefixed chunk blobs (each built with chunk_serialize),
+ * byte-identical by construction.  The batch is a client-only driver feature:
+ * it never crosses the wire, so there is no PROTOCOL_VERSION bump and no server
+ * change. */
+
+#define BATCH_MAGIC "FSTRESBATCH"
+#define BATCH_MAGIC_LEN 11
+#define BATCH_FORMAT_VERSION 1
+/* A single deserialized record is bounded by the chunk codec's 64 MB cap
+ * (the same per-file data cap chunk_deserialize enforces). */
+#define BATCH_MAX_RECORD (64ULL * 1024 * 1024)
+
+bool batch_write_header(int fd, const Config* config);
+bool batch_write_chunk(int fd, Chunk* chunk);
+int batch_read_apply(int fd, const Config* config, const char* dest_root);
+
+#endif