Add large-file (>1MB) OPL4 loading and fix track-end hang

- One-shot cold loader (asm/opl4_loader.asm, overlaid into plugin
  page 5) uploads embedded PCM sample blocks into ZXM-MoonSound SRAM
  and compacts the VGM stream in place, letting files over the normal
  64-page megabuffer budget play back through the existing pipeline.
- instant_abort() was filling only one of the two ISR command buffers
  with the shutdown chain; the chain (SILENCE_OPL + WAVE_OFF/TL +
  CMD_ISR_DONE) doesn't fit in one buffer, so after the first
  CMD_END_BUF the ISR swapped into stale data and never reached
  CMD_ISR_DONE, hanging at the end of long tracks instead of
  advancing to the next one. Now fills both buffers while frozen and
  syncs update_buffer()'s own ready-tracking state.
- Switch to 14MHz before the SRAM upload (previously ran at whatever
  speed WC's file I/O left active) and show "Loading..." for .vgz and
  files >= 1MB, since the busy-waited upload takes several seconds.
- Confirmed on real hardware: long OPL4 tracks (previously stuck
  looping at the end) now finish and advance normally, ESC exits
  cleanly.

src_sdcc/inc/variant_cfg.h

@@ -35,7 +35,7 @@
 /* Плотная раскладка: полная версия spectrum.s + загрузчик OPL4.
  * Проверяется build.bat: _CODE должен закончиться раньше _DATA,
  * а последняя секция — раньше 0xC000. */
-#define VARIANT_DATA_LOC      0xB8D0
+#define VARIANT_DATA_LOC      0xB8DC
 
 /* ── Спектроанализатор: скорости затухания (в TV-кадрах) ────────── */
 /* Меньше = быстрее затухание.  1 = каждый кадр (~20 мс).             */

src_sdcc/inc/vgm.h

@@ -188,6 +188,7 @@ extern uint8_t  vgm_chip_type;
 
 /** Массив обнаруженных чипов (для вывода информации о файле) */
 extern uint8_t          vgm_chip_count;
+extern uint8_t          vgm_opl4_preloaded_rom;
 extern vgm_chip_entry_t vgm_chip_list[VGM_MAX_CHIPS];
 
 /** Адрес точки петли (#C000–#FFFF); 0 = нет петли */