diagcat.inc is the single catalog: one line per diagnostic with a stable
number in a per-category range. Included by errors.inc it gives E_*
constants; included by diagnostics.s it gives the message table.
docs/diagnostics.md is generated from it. The build rejects out-of-range
or duplicate numbers and any grammar .expect symbol missing from it.

diagnostics.s prints a two-line, 40-column-safe screen form and appends
full lines to NAME.ERR. NAME.ERR is scratched at the start and created
only when needed. After 20 errors, E221 replaces the next one and the
caller must stop. diskio.s gains output-file and DOS-command wrappers.

Verified in x128: screen RAM rows, diag.err and limit.err read back from
the D81, stale-file scratching, an unwritable ERR name (DOS 33 -> E201
once), and no open files after any phase. A counting mutation is caught.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>