Fix TSan build: Add weak stubs for sanitizer compatibility

Added weak stubs to core/link_stubs.c for symbols that are not needed
in HAKMEM_FORCE_LIBC_ALLOC_BUILD=1 (TSan/ASan) builds:

Stubs added:
- g_bump_chunk (int)
- g_tls_bcur, g_tls_bend (__thread uint8_t*[8])
- smallmid_backend_free()
- expand_superslab_head()

Also added: #include <stdint.h> for uint8_t

Impact:
- TSan build: PASS (larson_hakmem_tsan successfully built)
- Phase 2 ready: Can now use TSan to debug Larson crashes

Next: Use TSan to investigate Larson 47% crash rate

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/link_stubs.c

@@ -1,5 +1,6 @@
 #include <stddef.h>
 #include <stdlib.h>
+#include <stdint.h>
 #include <sys/types.h>
 
 // Weak, no-op stubs to satisfy link in configurations where
@@ -34,3 +35,18 @@ __attribute__((weak)) int pool_reg_lookup(void* ptr, pid_t* tid_out, int* class_
 
 // Memory profile print stub (bench_comprehensive references this symbol)
 __attribute__((weak)) void hak_tiny_print_memory_profile(void) {}
+
+// TSan/ASan build stubs (avoid link errors when HAKMEM_FORCE_LIBC_ALLOC_BUILD=1)
+// These symbols are referenced in refill/bump allocation paths but not needed for sanitizer builds
+__attribute__((weak)) int g_bump_chunk = 1;
+__attribute__((weak)) __thread uint8_t* g_tls_bcur[8] = {0};
+__attribute__((weak)) __thread uint8_t* g_tls_bend[8] = {0};
+
+__attribute__((weak)) void smallmid_backend_free(void* ptr, int unused) {
+    (void)ptr; (void)unused;
+}
+
+__attribute__((weak)) int expand_superslab_head(void* head) {
+    (void)head;
+    return -1;  // Failure (not needed in sanitizer build)
+}