fix: resolve cppcheck warnings for const pointer and static functions

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

src/ringbuffer.h

@@ -13,8 +13,6 @@ typedef struct {
 
 int  ring_init(RingBuf *r, size_t capacity);
 void ring_destroy(RingBuf *r);
-size_t ring_readable(const RingBuf *r);
-size_t ring_writeable(const RingBuf *r);
 size_t ring_write(RingBuf *r, const float *data, size_t count);
 size_t ring_read(RingBuf *r, float *data, size_t count);