fix(buffer): remove const from char constructor to prevent UB

- Change: explicit buffer(char const & c) → explicit buffer(char & c)
- Remove dangerous const_cast in implementation
- Before: buffer(const_cast<char*>(&c), 1) [UB if c is truly const]
- After: buffer(&c, 1) [safe, requires non-const char]
- Prevents undefined behavior from modifying compile-time constants
- Constructor now correctly requires mutable char reference
- Aligns with buffer's mutable data semantics

The previous implementation with const_cast could lead to:
- Modifying string literals (undefined behavior)
- Modifying const variables (undefined behavior)
- Runtime crashes or data corruption

Example of prevented misuse:
  buffer buf('X');           // Now: compile error ✓
  char c = 'X';
  buffer buf(c);             // Now: works correctly ✓
  const char cc = 'Y';
  buffer buf(cc);            // Now: compile error ✓

include/libipc/buffer.h

@@ -24,7 +24,7 @@ public:
     explicit buffer(byte_t (& data)[N])
         : buffer(data, sizeof(data)) {
     }
-    explicit buffer(char const & c);
+    explicit buffer(char & c);
 
     buffer(buffer&& rhs);
     ~buffer();

src/libipc/buffer.cpp

@@ -46,8 +46,8 @@ buffer::buffer(void* p, std::size_t s)
     : buffer(p, s, nullptr) {
 }
 
-buffer::buffer(char const & c)
-    : buffer(const_cast<char*>(&c), 1) {
+buffer::buffer(char & c)
+    : buffer(&c, 1) {
 }
 
 buffer::buffer(buffer&& rhs)