fix(nrf): move the panel SPI off SPIM3 onto SPIM2 (anomalies 195/198)

[davelee98]

Single-commit nRF52840 change: move the panel SPI off SPIM3 and onto SPIM2. Independent of #135 and #136.

Why

SPIM3 is the only nRF52840 SPIM instance carrying anomalies 195 and 198, and both bite this firmware:

• 195 — the instance keeps drawing current after disable. The nrfx workaround pokes 0x4002F004 on every uninit and never undoes it on re-init.
• 198 — transmit data may be corrupted when another bus master touches the TX buffer's RAM block mid-transfer. The Arduino per-byte path makes that buffer a one-byte stack local, contended by SoftDevice interrupts on every byte. Worse, 198's workaround is compiled out here entirely, because NRFX_SPIM3_NRF52840_ANOMALY_198_WORKAROUND_ENABLED is never defined.

Both nrfx workarounds are gated on p_spim == NRF_SPIM3, so moving to SPIM2 makes the anomalies inapplicable rather than worked around.

How

The Adafruit core binds the global SPI object to an instance chosen by SPI_32MHZ_INTERFACE (framework SPI.cpp): 0 — its default — gives SPIM3, 1 gives SPIM2. Only SPI matters: bb_epaper drives the panel through that global, and nothing here ever calls SPI1.begin(), so the instance handed to SPI1 is left uninitialised.

The flag is scoped to [env:nrf52840custom], so it does not reach the ESP32 environments through [env] inheritance.

Cost

None at our clock. SPIM2 tops out at 8 MHz and bbepInitIO already asks for exactly 8000000, which setClockDivider maps to NRF_SPIM_FREQ_8M on either instance. It forfeits the 16/32 MHz only SPIM3 offers — deliberately, since raising the panel clock increases the DMA bus pressure this change exists to avoid.

No pin change (PIN_SPI_* untouched). No I2C conflict: Wire is TWIM0 and Wire1 is TWIM1, while SPIM2's block is shared only with SPIS2/SPI2, neither used.

Verification

The macro is consumed by the framework's SPI.cpp, not by our sources, so it only works while -D reaches the framework build — and if it silently stops doing so we are back on SPIM3 with no error. Confirmed for this commit:

$ pio run -e nrf52840custom -v | grep 'SPI_32MHZ_INTERFACE=1.*SPI\.cpp'
-DSPI_32MHZ_INTERFACE=1 … framework-arduinoadafruitnrf52-seeed/libraries/SPI/SPI.cpp
  -o .pio/build/nrf52840custom/lib85a/SPI/SPI.cpp.o

The check is recorded in an inline comment so it survives a platform update. Builds clean; not yet exercised on hardware — it wants a panel-refresh soak before merge.