Fix self-referential writecopy after buffer growth

[carrerasdarren-cell]

Summary

• Preserve a writecopy source that points into the destination buffer when
  growing that buffer moves its allocation.
• Add self-referential copy coverage for both bit orders and for aligned and
  unaligned destinations.

Problem

oggpack_writecopy_helper saves its source pointer, expands b->buffer with
realloc, and then consumes the saved pointer. If the source points into
b->buffer and growth moves the allocation, the aligned memmove or the
unaligned byte loop reads freed storage.

A 192-byte self-copy that grows the initial 256-byte buffer reliably reports:

ERROR: AddressSanitizer: heap-use-after-free
READ of size 192
    #1 oggpack_writecopy_helper bitwise.c:214
freed by thread T0 here:
    #1 oggpack_writecopy_helper bitwise.c:200

The patch records an internal source's byte offset before growth and rebases
it after a successful realloc. External sources are unchanged.

Validation

• Autotools make check with Clang -Wall -Wextra -Werror: 2/2 pass
• CMake Release test_bitwise and test_framing: pass
• CMake ASan/UBSan test_bitwise and test_framing: pass
• Standalone reproducer under ASan/UBSan: fails before, passes after
• Strict C89 build with -pedantic-errors -Wall -Wextra -Werror: pass
• Clang static analyzer: no findings