test: coverage for --checksum-choice/--cc and --checksum-seed

Unit: digest selection with known vectors (xxh64 seed 0/empty, md5 RFC vectors,
seed changes xxh64 but not md5, name mapping, bounded buffer rejection), CLI
parse (all spellings, = forms, --cc alias, unsupported rejected, seed strict
decimal), config wire round-trip and out-of-range algo rejection, and seeded
delta signature/compute symmetry (matching seed rebuilds, mismatched seed yields
no block matches). Integration: unchanged skip + same-size/mtime redetect for
xxh64 and md5 with and without -m, near-zero-data skip run, deterministic seed,
and a byte-exact seeded delta run.

tests/runner.c

@@ -1,6 +1,7 @@
 #include "test_array_list.h"
 #include "test_chunk.h"
 #include "test_change_list.h"
+#include "test_checksum.h"
 #include "test_client_cli.h"
 #include "test_compression.h"
 #include "test_config.h"
@@ -46,6 +47,7 @@ int main() {
   RUN_TEST(test_config);
   RUN_TEST(test_compression);
   RUN_TEST(test_scanner);
+  RUN_TEST(test_checksum);
   RUN_TEST(test_delta);
   RUN_TEST(test_data);
   RUN_TEST(test_protocol);