Add missing C: (composer), R: (rhythm) and X: fields and return error (#21)

* Add configuration for CMake build system alongside autoconf

- Add a modern CMake build system (`CMakeLists.txt`, `CMakePresets.json`) that coexists with the legacy
autoconf/Makefile build
- Shared source files (`midifile.c`, `parseabc.c`, `music_utils.c`, `parser2.c`) are compiled once via OBJECT
libraries and linked into the 8 binaries
- Three presets: `default` (Release), `debug`, `sanitize` (ASan + UBSan)
- Generates `compile_commands.json` for clangd/LSP editor support
- Install rules match the legacy Makefile (binaries, doc files, man pages)
- Pinned to `-std=gnu89` because the codebase mixes K&R `()` and ANSI typed prototypes — in C23/gnu23 (GCC 15+
default), `()` means `(void)`, making these a hard error. Note: **the existing autoconf build is also broken with
GCC 15** for the same reason

```sh
cmake --preset debug
cmake --build --preset debug
cmake --install build/debug --prefix /usr/local

Documentation

- README.md: added Building section with both autoconf and CMake instructions
- doc/readme.txt: added build instructions in the existing preamble
- doc/CHANGES: added changelog entry

Test plan

- All 3 presets configure and build with GCC 15
- Smoke test: abc2midi samples/coleraine.abc produces valid MIDI through mftext
- Sanitizer build (--preset sanitize) runs clean on sample files
- Install layout verified: 8 binaries, 10 doc files, 8 man pages in correct paths
- Build on macOS (untested, should work with AppleClang)

* Implement basic testing infrastructure

The CMake build includes a test suite covering all 8 programs:

- **Smoke tests** verify each binary runs cleanly with `-ver`.
- **Golden-file tests** run each program on a sample input and compare the
  (normalized) output to a checked-in reference. Binary MIDI outputs are
  piped through `mftext` to produce diffable text. Volatile lines (version
  banners, dates, temporary paths) are stripped before comparison.

```sh
ctest --preset debug

ctest --preset debug -L golden
ctest --preset debug -L smoke
```

To regenerate the golden files after an intentional behavioural change,
review the diff, then commit:

```sh
cmake --build build/debug --target update-golden
git diff tests/golden/
```

* Factorize more the test CMake code

* Add GitHub Action to run the CI and output a status badge

* Add GitHub Action workflow dispatch to allow running from the UI

* Now abc2midi exits 1 if an error occurs

Before it was always returning 0, hiding failures complicated to track in
complex build systems.

* Output missing C:, R: and X: headers as MIDI text meta-events

* Update changelog and comments

---------

Co-authored-by: Seymour Shlien <fy733@ncf.ca>

genmidi.c

@@ -3168,6 +3168,13 @@ long writetrack(int xtrack)
                           strlen(atext[pitch[j]]));
       };
       break;
+    case COMPOSER: /* [RK] 2026-03-31 */
+      if (texton) {
+        char cbuf[300];
+        snprintf(cbuf, sizeof(cbuf), "C:%s", atext[pitch[j]]);
+        mf_write_meta_event(0L, text_event, cbuf, strlen(cbuf));
+      }
+      break;
     case TITLE:
 /*  Write name of song as sequence name in track 0 and as track 1 name. */
 /*  karaokestarttrack routine handles this instead if tune is a Karaoke tune. */