feat: implement .tmp directory isolation for safe hot-reload

Add foundation for cross-platform hot-reload system by isolating
loaded modules from source files using a temporary directory.

Changes:
- Add configurable tmp directory parameter to stk_init()
  (defaults to mods/.tmp/ if not specified)
- Copy all modules from mods/ to .tmp/ on initialization
- Load modules exclusively from .tmp/ directory
- Clean up .tmp/ directory on shutdown
- Add cross-platform file operations:
  * platform_mkdir() - create directories
  * platform_copy_file() - copy files
  * platform_remove_file() - delete files
  * platform_remove_dir() - delete directory and contents
- Improve BSD kqueue implementation to detect file overwrites
  (adds individual file watches with NOTE_WRITE)

This isolates the loaded shared libraries from source files,
preventing segfaults when users overwrite mods using cp/copy
operations. The actual reload logic remains unimplemented
(marked as TODO in stk_poll switch cases).

include/stk.h

@@ -19,7 +19,7 @@ typedef enum {
 	STK_MOD_RELOAD
 } stk_module_event_t;
 
-int stk_init(const char *mod_dir);
+int stk_init(const char *mod_dir, const char *tmp_dir);
 void stk_shutdown(void);
 size_t stk_module_count(void);
 size_t stk_poll(void);