Implement basic testing infrastructure (#19)

* 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

CMakePresets.json

@@ -0,0 +1,66 @@
+{
+  "version": 6,
+  "cmakeMinimumRequired": { "major": 3, "minor": 25, "patch": 0 },
+  "configurePresets": [
+    {
+      "name": "default",
+      "displayName": "Default (Release)",
+      "binaryDir": "${sourceDir}/build/${presetName}",
+      "cacheVariables": {
+        "CMAKE_BUILD_TYPE": "Release"
+      }
+    },
+    {
+      "name": "debug",
+      "displayName": "Debug",
+      "binaryDir": "${sourceDir}/build/${presetName}",
+      "cacheVariables": {
+        "CMAKE_BUILD_TYPE": "Debug"
+      }
+    },
+    {
+      "name": "sanitize",
+      "displayName": "Debug + Sanitizers (ASan + UBSan)",
+      "binaryDir": "${sourceDir}/build/${presetName}",
+      "cacheVariables": {
+        "CMAKE_BUILD_TYPE": "Debug",
+        "ABCMIDI_SANITIZERS": "ON"
+      }
+    }
+  ],
+  "buildPresets": [
+    {
+      "name": "default",
+      "configurePreset": "default"
+    },
+    {
+      "name": "debug",
+      "configurePreset": "debug"
+    },
+    {
+      "name": "sanitize",
+      "configurePreset": "sanitize"
+    }
+  ],
+  "testPresets": [
+    {
+      "name": "default",
+      "configurePreset": "default",
+      "output": { "outputOnFailure": true }
+    },
+    {
+      "name": "debug",
+      "configurePreset": "debug",
+      "output": { "outputOnFailure": true }
+    },
+    {
+      "name": "sanitize",
+      "configurePreset": "sanitize",
+      "output": { "outputOnFailure": true },
+      "environment": {
+        "ASAN_OPTIONS": "abort_on_error=1:halt_on_error=1",
+        "UBSAN_OPTIONS": "abort_on_error=1:halt_on_error=1:print_stacktrace=1"
+      }
+    }
+  ]
+}