symbol.c: implement adaptive symbol table for memory efficiency

Replace fixed 256-element hash array in mrb_state with adaptive approach:
- Linear search for <=255 symbols (typical embedded use case)
- Hash table allocated on-demand when symbols exceed threshold
- Reduces mrb_state size by 1KB per instance (1068->36 bytes in symbol fields)
- Configurable threshold via MRB_SYMBOL_LINEAR_THRESHOLD in mrbconf.h

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby.h

@@ -155,6 +155,7 @@ typedef uint8_t mrb_code;
 typedef uint32_t mrb_aspec;
 
 typedef struct mrb_irep mrb_irep;
+
 struct mrb_state;
 
 #ifndef MRB_FIXED_STATE_ATEXIT_STACK_SIZE
@@ -281,10 +282,8 @@ typedef struct mrb_state {
 
   mrb_sym symidx;
   const char **symtbl;
-  uint8_t *symlink;
-  uint8_t *symflags;
-  mrb_sym symhash[256];
   size_t symcapa;
+  struct mrb_sym_hash_table *symhash;
 #ifndef MRB_USE_ALL_SYMBOLS
   char symbuf[8];                         /* buffer for small symbol names */
 #endif