fix(spi): correct target mode and bit-order handling

[floitsch]

Stacked on #123.

This fixes target-mode issues found while bringing up asynchronous SPI target support in Toit:

• Map the public target-facing TX/RX LSB-first flags to the corresponding peripheral register directions. Classic ESP32 has the opposite internal naming; newer chips use the target-facing direction directly.
• On classic ESP32 without DMA, select the correct MOSI sampling edge in modes 1 and 3. Keep the existing DMA settings because that DMA engine depends on them.
• On ESP32-S3 in mode 2, select separate DMA/non-DMA internal edges and make the first non-DMA output bit valid before the leading controller edge.

Validated with exhaustive two-device hardware tests on both classic ESP32 and ESP32-S3: modes 0 through 3, independent RX/TX bit order, supported DMA and non-DMA direction combinations, 50 kHz through 2 MHz, early termination, one-way transfers, and sizes through 4092 bytes. The classic mode 1/3 issue corresponds to Espressif IDFGH-6011.

[floitsch]

@CodeRabbit review

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Walkthrough

Changes

SPI slave corrections

Layer / File(s)	Summary
Target-specific bit-order mapping components/esp_driver_spi/src/gpspi/spi_slave.c	ESP32 targets now map RX and TX LSB-first flags inversely. Other targets retain the existing mapping.
DMA-aware slave clock edges components/hal/esp32/include/hal/spi_ll.h, components/hal/esp32s3/include/hal/spi_ll.h	Slave modes 1 and 3 on ESP32, and mode 2 on ESP32-S3, now select clock edges based on DMA usage. ESP32-S3 also enables rsck_data_out for non-DMA mode 2.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the primary SPI target-mode and bit-order fixes in the changeset.
Description check	✅ Passed	The description accurately explains the SPI fixes, target-specific behavior, and validation performed.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch floitsch/spi-target-fixes

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