abc2midi: fix -PMAR perturbing note timing and end-of-track time (#26)

Emitting a Marker meta-event from the PART case wrote the event with
delta_time_track0 (correct, since that's the conductor-track accumulator)
but did not also reset delta_time. On track 0 in multi-track mode,
delta_time also accumulates via timestep() and is what writetrack()
returns to the MIDI library as the end-of-track delta — so each marker
left a stale delta_time that pushed the end-of-track event past the end
of the music. In single-track mode (ntracks == 1), markers were written
with delta_time_track0 too, but delta_time is the only relevant counter
in that mode, so notes following each marker were shifted later.

Fix: mirror the TEMPO handler — on ntracks == 1 use delta_time and reset
it; on ntracks != 1 keep using delta_time_track0 for the event delta but
also zero delta_time so the end-of-track is not inflated.

Reported by James Allwright with the partdemo.abc test case (now in
samples/), which exercises the parts != -1 path (header P:BACDBAC plus
body P: labels A/B/C/D). James independently fixed the same bug in his
abc2midiu fork in r32 (commit 34b7c32, "Add -PMAR option for part
markers"), where the equivalent reset is on the single delta_time
counter — his fork having retired delta_time_track0 in an earlier
refactor. Verified that mftext output of the generated MIDI is now
byte-identical to the non-PMAR output except for the added Marker
events, on both partdemo.abc (single-track) and samples/demo.abc tune 5
with P:(AB)3 (multi-track).

Add a CMake/CTest regression test (abc2midi_pmar_partdemo) that locks in
the post-fix mftext output. To support it, add_golden_test() gains an
optional NAME (to register multiple tests against the same TYPE+SAMPLE
pair) and an optional ABC2MIDI_ARGS (forwarded to the abc2midi
invocation in run_via_mid). Reverting the genmidi.c fix makes the new
test fail; reapplying it makes it pass.

tests/CMakeLists.txt

@@ -38,12 +38,23 @@ foreach(bin IN LISTS ABCMIDI_BINARIES)
   list(APPEND BINARY_DEFS "-D${BIN_UPPER}=$<TARGET_FILE:${bin}>")
 endforeach()
 
-# Helper: register a golden test for one program against one sample
+# Helper: register a golden test for one program against one sample.
+#
+# Optional NAME overrides the default test name (${TYPE}_${stem}); use this
+# when registering multiple tests against the same TYPE+SAMPLE pair (e.g. a
+# plain abc2midi run and a -PMAR run on the same input).
+#
+# Optional ABC2MIDI_ARGS forwards extra arguments to the abc2midi invocation
+# (only meaningful for TYPEs that go ABC -> MIDI -> diff).
 function(add_golden_test)
-  cmake_parse_arguments(T "" "TYPE;SAMPLE" "" ${ARGN})
+  cmake_parse_arguments(T "" "TYPE;SAMPLE;NAME" "ABC2MIDI_ARGS" ${ARGN})
 
-  get_filename_component(stem "${T_SAMPLE}" NAME_WE)
-  set(test_name "${T_TYPE}_${stem}")
+  if(T_NAME)
+    set(test_name "${T_NAME}")
+  else()
+    get_filename_component(stem "${T_SAMPLE}" NAME_WE)
+    set(test_name "${T_TYPE}_${stem}")
+  endif()
 
   add_test(
     NAME "${test_name}"
@@ -52,6 +63,7 @@ function(add_golden_test)
       -DSAMPLE=${SAMPLES_DIR}/${T_SAMPLE}
       -DGOLDEN=${GOLDEN_DIR}/${test_name}.txt
       -DTMPDIR=${TEST_TMPDIR}
+      "-DABC2MIDI_ARGS=${T_ABC2MIDI_ARGS}"
       ${BINARY_DEFS}
       -P "${CMAKE_CURRENT_SOURCE_DIR}/run_test.cmake"
   )
@@ -67,6 +79,17 @@ endforeach()
 add_golden_test(TYPE abc2midi SAMPLE demo.abc)
 add_golden_test(TYPE abc2abc  SAMPLE demo.abc)
 
+# Regression test for the -PMAR option (Part Marker meta-events).
+# Locks in marker emission and verifies the markers do not perturb note
+# timing or end-of-track time (bug fixed by carrying delta_time alongside
+# delta_time_track0 when emitting markers on the conductor track).
+add_golden_test(
+  TYPE          abc2midi
+  SAMPLE        partdemo.abc
+  NAME          abc2midi_pmar_partdemo
+  ABC2MIDI_ARGS -PMAR
+)
+
 # --- Regenerate-golden convenience target ---
 #
 # Usage:  cmake --build build/debug --target update-golden