Worker side of the libqemu picsimlab_spi_event_batch / CS-gating change:
- _on_spi_batch(): replay a whole SPI transfer in bulk (custom-chip runtime,
then ePaper feed, then the spi_batch buffer) instead of one _on_spi_event per
byte. Registered as a trailing _SPI_BATCH field of _CallbacksT.
- _sync_cs_events(): disable SPI chip-select callbacks for pure-display sims,
enable them when an ePaper / custom-chip SPI slave is registered (no-op on
older libqemu without qemu_picsimlab_enable_spi_cs_events).
- _on_pin_change(): flush the SPI batch before each gpio_change so the byte
stream stays ordered against the DC pin now that CS no longer triggers the
flush.
Backward compatible: an older libqemu never calls the batch callback or the CS
setter, so it just keeps the per-byte path. esp32-doom: 0.04 -> ~1.0-1.5 FPS
wall-clock (~26-37x), render verified correct.
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