Unify package version on VERSION file; bump abc2midi to 5.03 (#25)

- abc2midi 5.03 (April 2026): bumped #define VERSION in store.c to reflect this branch's accumulated fixes
(error exit status, missing C:/R:/X: header emission, %%MIDIdef macros allowed anywhere) plus the
build-system additions.
- Single source of truth for the package release date: the VERSION file is now consumed by both build
paths.
  - CMakeLists.txt reads it via file(STRINGS VERSION ABCMIDI_VERSION ...); the previously hard-coded
project(VERSION 2026.02.24) (an unused PROJECT_VERSION) is dropped because the date format isn't
MAJOR.MINOR.PATCH.
  - configure.ac uses m4_esyscmd_s([cat VERSION]) in AC_INIT, replacing the very stale hard-coded
2011-08-03.
  - The two build paths remain independent — no cross-generation between CMake and autoconf.
- Test normalization fix (tests/run_test.cmake): the version-banner regex now requires a capitalized month
and accepts a date with or without a day number. The previous regex was lowercase-permissive and silently
ate PostScript lines like 0.5 setlinewidth 0 0 moveto in yaps output — tests/golden/yaps_coleraine.txt is
regenerated to reflect the now-correctly-preserved PostScript.
- Docs:
  - doc/CHANGES: April 25 2026 entry covering the version bump and VERSION-file unification.
  - doc/readme.txt: per-tool listing updated (abc2midi 5.03 April 2026, midistats 1.03 February 20 2026
synced to source).
  - doc/abc2midi.1: header label bumped to 5.03 April 2026 (content not audited).
  - README.md: new Maintainers / releasing section documenting the release procedure, including the
requirement to run autoreconf -f so the committed configure picks up the new AC_INIT arguments.

doc/CHANGES

@@ -15724,3 +15724,19 @@ files (midifile.c, parseabc.c, music_utils.c, parser2.c) are compiled
 once via OBJECT libraries.  The build exports compile_commands.json
 for LSP/clangd support.  Note: the codebase requires -std=gnu89 until
 the K&R-to-ANSI prototype migration is completed.
+
+April 25 2026 [RK]
+
+abc2midi: bumped to version 5.03 (April 2026), reflecting the recent
+fixes (error exit status, missing C:/R:/X: headers emitted as MIDI
+text meta-events, %%MIDIdef macros allowed anywhere) and the build
+system additions on this branch.
+
+build: the package release date in the VERSION file is now the single
+source of truth.  CMakeLists.txt reads it into ABCMIDI_VERSION (no
+longer hard-coded as 2026.02.24 in project()), and configure.ac uses
+m4_esyscmd_s([cat VERSION]) so AC_INIT picks it up at autoreconf time
+(was hard-coded as the very stale 2011-08-03).  Maintainers must run
+"autoreconf -f" before tagging a release so the committed configure
+script reflects the new VERSION; see the Releasing section in
+README.md.