feat: add delta transfer for incremental sync

Implement rsync-style delta transfer using rolling checksums (Adler-32 +
xxHash32). When a file exists on both sides but has changed, only the
changed blocks are transmitted instead of the entire file.

- New status codes: STATUS_DELTA_SIGNATURE, STATUS_DELTA_DATA
- Protocol version bumped to 1.2.0
- Server generates block signature, client computes delta
- Auto-fallback to whole-file when delta >= 70% of file size
- Works with zstd compression on delta stream
- Configurable block size (default 8KB, --delta-block flag)
- 13 unit tests covering hashing, signature roundtrip, delta compute/apply,
  file growth/shrink, and decision logic

src/shared/delta.h

@@ -0,0 +1,81 @@
+#ifndef DELTA_H
+#define DELTA_H
+
+#include "data.h"
+#include <stdbool.h>
+#include <stdint.h>
+#include <stddef.h>
+
+#define DELTA_BLOCK_SIZE_DEFAULT  8192U
+#define DELTA_BLOCK_SIZE_MIN      1024U
+#define DELTA_BLOCK_SIZE_MAX      65536U
+#define DELTA_MIN_FILE_SIZE       16384ULL
+#define DELTA_MAX_FILE_SIZE       (256ULL * 1024 * 1024)
+#define DELTA_MAX_SIZE_RATIO      10.0
+#define DELTA_FALLBACK_RATIO      0.7
+#define DELTA_ADLER32_MODULUS     65521U
+
+#define DELTA_OP_BLOCK_MATCH  0x01
+#define DELTA_OP_LITERAL      0x02
+
+typedef struct {
+  uint32_t adler32;
+  uint32_t xxhash;
+} DeltaBlockSig;
+
+typedef struct {
+  uint64_t file_size;
+  uint32_t block_size;
+  uint32_t block_count;
+  DeltaBlockSig *blocks;
+} DeltaSignature;
+
+typedef enum {
+  DELTA_INSTR_BLOCK_MATCH = 0x01,
+  DELTA_INSTR_LITERAL     = 0x02
+} DeltaInstrType;
+
+typedef struct {
+  DeltaInstrType type;
+  union {
+    struct {
+      uint32_t block_index;
+      uint32_t block_offset;
+      uint32_t length;
+    } match;
+    struct {
+      uint8_t *data;
+      uint32_t length;
+    } literal;
+  };
+} DeltaInstruction;
+
+typedef struct {
+  uint64_t new_file_size;
+  uint32_t instruction_count;
+  DeltaInstruction *instructions;
+  uint64_t delta_size;
+} Delta;
+
+DeltaSignature *delta_signature_create(const void *old_file_data,
+                                        uint64_t old_file_size,
+                                        uint32_t block_size);
+Data *delta_signature_serialize(const DeltaSignature *sig);
+DeltaSignature *delta_signature_deserialize(const Data *data);
+void delta_signature_destroy(DeltaSignature *sig);
+
+Delta *delta_compute(const void *new_file_data, uint64_t new_file_size,
+                      const DeltaSignature *sig, uint32_t block_size);
+Data *delta_serialize(const Delta *delta);
+Delta *delta_deserialize(const Data *data);
+void *delta_apply(const void *old_data, uint64_t old_size, const Delta *delta,
+                  uint32_t block_size);
+void delta_destroy(Delta *delta);
+
+bool delta_should_attempt(uint64_t old_size, uint64_t new_size);
+bool delta_is_worthwhile(const Delta *delta, uint64_t new_file_size);
+
+uint32_t delta_adler32(const void *data, uint32_t len);
+uint32_t delta_xxhash32(const void *data, uint32_t len);
+
+#endif