fix(ESP32 RMT): make RMT memory block size user-overridable (fixes BLE/WiFi ghost pixels)

[darkgrue]

Problem

When a BLE or WiFi connection is active on ESP32, sparse LED patterns produce ghost pixels at index N+8 for every addressed pixel N. The defect scales with strip position and is introduced during RMT transmission, not in the software color buffer.

Root cause: config.mem_block_num = 1 allocates a single 64-symbol RMT memory block. With ping-pong DMA, the RMT refill ISR must be serviced within ~40 µs at WS2812x 800 Kbps. NimBLE and WiFi event handlers run at higher interrupt priority than the RMT refill ISR and routinely preempt for longer, leaving the data line idle past the ~300 µs LED latch threshold.

Fixes #921.

Solution

Add a user-overridable preprocessor macro NEOPIXELBUS_RMT_MEM_BLOCK_SYMBOLS that controls the number of RMT memory blocks allocated.

Default (backward compatible): 64 symbols → mem_block_num = 1 (unchanged behavior)

BLE/WiFi fix: add to build flags:

-D NEOPIXELBUS_RMT_MEM_BLOCK_SYMBOLS=512

This allocates 8 memory blocks; the ping-pong half-buffer holds 256 symbols × 1.25 µs = 320 µs, which exceeds the 300 µs latch threshold.

Changes

src/internal/methods/NeoEsp32RmtMethod.h:

• Added #ifndef NEOPIXELBUS_RMT_MEM_BLOCK_SYMBOLS / #define NEOPIXELBUS_RMT_MEM_BLOCK_SYMBOLS 64 / #endif after the NEOPIXELBUS_RMT_INT_FLAGS block
• Changed config.mem_block_num = 1; → config.mem_block_num = (NEOPIXELBUS_RMT_MEM_BLOCK_SYMBOLS + 63) / 64;

Hardware Constraints

On the original ESP32 (8 RMT channels × 64 symbols each), setting NEOPIXELBUS_RMT_MEM_BLOCK_SYMBOLS=512 consumes all 8 channels' memory for one strip. Users running multiple simultaneous RMT strips must choose a value that fits within available channels. Recommended values by use case:

Scenario	Recommended value	mem_block_num
No BLE/WiFi (default)	64	1
BLE or WiFi active (ESP32)	512	8
BLE or WiFi active (S2/S3/C3, 4 ch)	256	4

Testing

Verified by the issue reporter's reproduction sketch (ESP32 + NimBLE server, single pixel set to red):

• Without fix: pixel 0 and pixel 8 both light red with a BLE client connected
• With -D NEOPIXELBUS_RMT_MEM_BLOCK_SYMBOLS=512: only pixel 0 lights