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/client/client_validation.c

@@ -8,7 +8,34 @@
 
 /* Validate config after parsing. Returns true if valid. */
 bool validate_config(const Config* config) {
-  if (!config->send_directory || !config->receive_root_directory) {
+  /* Phase 6 residual-batch modes relax the normal source+destination pair: the
+     batch driver is local and needs only what it consumes.  --only-write-batch
+     emits a batch from the source (no destination, no server);
+     --read-batch applies a batch to the destination (no source, no server);
+     --write-batch runs the live transfer AND emits a batch, so it keeps the
+     full pair. */
+  bool write_batch = config->write_batch != NULL;
+  bool only_write_batch = config->only_write_batch != NULL;
+  bool read_batch = config->read_batch != NULL;
+  if ((write_batch && only_write_batch) || (write_batch && read_batch) ||
+      (only_write_batch && read_batch)) {
+    log_message(LOG_LEVEL_ERROR,
+                "--write-batch, --only-write-batch, and --read-batch are mutually exclusive");
+    return false;
+  }
+  if (read_batch) {
+    if (!config->receive_root_directory) {
+      log_message(LOG_LEVEL_ERROR, "--read-batch requires a destination directory");
+      print_usage();
+      return false;
+    }
+  } else if (only_write_batch) {
+    if (!config->send_directory) {
+      log_message(LOG_LEVEL_ERROR, "--only-write-batch requires a source directory");
+      print_usage();
+      return false;
+    }
+  } else if (!config->send_directory || !config->receive_root_directory) {
     log_message(LOG_LEVEL_ERROR, "source and destination directories are required");
     print_usage();
     return false;