fix: resolve cppcheck warnings for const pointer and static functions

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

src/ringbuffer.c

@@ -30,7 +30,7 @@ void ring_destroy(RingBuf *r) {
   r->capacity = 0;
 }
 
-size_t ring_readable(const RingBuf *r) {
+static size_t ring_readable(const RingBuf *r) {
   size_t h = load_head(r);
   size_t t = load_tail(r);
   if (h >= t)
@@ -39,7 +39,7 @@ size_t ring_readable(const RingBuf *r) {
     return r->capacity - (t - h);
 }
 
-size_t ring_writeable(const RingBuf *r) {
+static size_t ring_writeable(const RingBuf *r) {
   return r->capacity - 1 - ring_readable(r);
 }