[PATCH] Fix float16 pack/unpack on big-endian systems

PyFloat_Pack2() and PyFloat_Unpack2() (and their underscore-prefixed
predecessors) take an 'le' parameter where 0 means big-endian and 1
means little-endian storage. The code was passing PY_BIG_ENDIAN which
evaluates to 0 on little-endian and 1 on big-endian - exactly the
opposite of what is needed, since the bitstream always operates in
big-endian (MSB-first) byte order.

On little-endian systems PY_BIG_ENDIAN=0 happened to produce the correct
result (big-endian storage), masking the bug. On big-endian systems
PY_BIG_ENDIAN=1 caused little-endian storage, resulting in byte-swapped
float16 values.

Replace PY_BIG_ENDIAN with the literal 0 (= big-endian) to get
consistent behavior across all architectures.

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Assisted-by: Claude (Anthropic) <https://claude.ai>

Author: lemenkov

src/bitstruct/c.c

@@ -430,11 +430,11 @@ static void pack_float_16(struct bitstream_writer_t *self_p,
 #if PY_VERSION_HEX >= 0x030B00A7
     PyFloat_Pack2(PyFloat_AsDouble(value_p),
                   (char*)&buf[0],
-                  PY_BIG_ENDIAN);
+                  0);
 #else
     _PyFloat_Pack2(PyFloat_AsDouble(value_p),
                    &buf[0],
-                   PY_BIG_ENDIAN);
+                   0);
 #endif
     bitstream_writer_write_bytes(self_p, &buf[0], sizeof(buf));
 }
@@ -447,9 +447,9 @@ static PyObject *unpack_float_16(struct bitstream_reader_t *self_p,
 
     bitstream_reader_read_bytes(self_p, &buf[0], sizeof(buf));
 #if PY_VERSION_HEX >= 0x030B00A7
-    value = PyFloat_Unpack2((const char*)&buf[0], PY_BIG_ENDIAN);
+    value = PyFloat_Unpack2((const char*)&buf[0], 0);
 #else
-    value = _PyFloat_Unpack2(&buf[0], PY_BIG_ENDIAN);
+    value = _PyFloat_Unpack2(&buf[0], 0);
 #endif
 
     return (PyFloat_FromDouble(value));