fix(core)!: enforce strict C89 compliance

BREAKING CHANGE: Public API now uses unsigned char instead of uint8_t

- Remove stdint.h dependency (C99 feature, not C89, I am a fucking idiot)
- Replace uint8_t with unsigned char throughout codebase
- Affects stk_init() return type and internal functions
- Corrects unintended C99 dependency, restoring intended C89 compliance

README.md

@@ -156,7 +156,7 @@ stk_init();
 ### API Reference
 
 #### Initialization
-- `uint8_t stk_init(void)` - Initialize stk, returns `STK_INIT_SUCCESS` on success
+- `unsigned char stk_init(void)` - Initialize stk, returns `STK_INIT_SUCCESS` on success
 - `void stk_shutdown(void)` - Shutdown and cleanup all modules
 
 #### Runtime