Fix misleading persisted_length comments in SDK

The comments implied persisted_length was an exact fixed storage
size. Based on experimentation with Claude, it is actually a maximum
buffer ceiling — the server persists only as many bytes as the encode
function writes via its length output parameter (already documented
correctly on vef_encode_func_t).

Updated both sdk/ and stable_sdk/v1/.

AI=CLAUDE
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: villagesql-adam

villagesql/sdk/include/villagesql/abi/types.h

@@ -538,7 +538,8 @@ typedef struct {
   // Encoded using UTF-8
   const char *name;
 
-  // Size of the binary representation when stored
+  // Maximum buffer size for binary storage; actual bytes persisted are
+  // determined by the encode function via its length output parameter.
   int64_t persisted_length;
 
   // Maximum size of the string representation (for decode output buffer)

villagesql/sdk/include/villagesql/extension.h

@@ -112,7 +112,7 @@
 // ending with .build():
 //
 //   make_type("mytype")
-//     .persisted_length(8)           // Bytes when stored
+//     .persisted_length(8)           // Max storage buffer (encode sets actual length)
 //     .max_decode_buffer_length(64)  // Max bytes for string representation
 //     .encode(&mytype_encode)        // String -> binary
 //     .decode(&mytype_decode)        // Binary -> string

villagesql/stable_sdk/v1/include/villagesql/abi/types.h

@@ -504,7 +504,8 @@ typedef struct {
   // Encoded using UTF-8
   const char *name;
 
-  // Size of the binary representation when stored
+  // Maximum buffer size for binary storage; actual bytes persisted are
+  // determined by the encode function via its length output parameter.
   int64_t persisted_length;
 
   // Maximum size of the string representation (for decode output buffer)

villagesql/stable_sdk/v1/include/villagesql/extension.h

@@ -112,7 +112,7 @@
 // ending with .build():
 //
 //   make_type("mytype")
-//     .persisted_length(8)           // Bytes when stored
+//     .persisted_length(8)           // Max storage buffer (encode sets actual length)
 //     .max_decode_buffer_length(64)  // Max bytes for string representation
 //     .encode(&mytype_encode)        // String -> binary
 //     .decode(&mytype_decode)        // Binary -> string