Flag collision in cbf.h

[dwpaley]

MSG_DIGESTWARN (a read-mode flag) and PAD_1K (write-mode) share a common bit 0x0020. This doesn't appear to be causing any problems, but it could be a latent headache and appears easy enough to fix. Claude will give the details:

Affected: CBFlib 0.9.8 (and current HEAD) — include/cbf.h:637-640

Two distinct header flags are #defined with the same value:

#define MSG_DIGESTWARN  0x0020   /* warn on digest mismatch — used on the READ path  */
#define PAD_1K          0x0020   /* pad binary section with 1023 nulls — WRITE path  */

PAD_2K (0x0040) and PAD_4K (0x0080) do not currently collide; only PAD_1K aliases MSG_DIGESTWARN.

Impact — latent, not a functional bug in normal use

MSG_DIGESTWARN is tested against file->read_headers (read path) and PAD_1K against file->write_headers (write path, src/cbf_write_binary.c:330-336). These are separate words, so ordinary read/write flows are unaffected. But the shared numeric value is a footgun: any code that builds or inspects a combined flag word, reuses one value across read+write, or reasons about the flags as one family (as the documentation does) will conflate "warn-on-digest" with "1 KB padding."

Suggested fix

Reassign PAD_1K to a free bit not used by the MIME_*/MSG_* read flags (mind the CBF_PARSE_* bits starting at 0x0100), or, if the read/write separation is intentional, document it explicitly at the definitions.