furi_hal_spi: take const uint8_t* tx_buffer in trx_dma()

The DMA path only reads from tx_buffer; nothing inside writes through
the pointer. Mirrors the signature of furi_hal_spi_bus_trx() which
already takes const uint8_t* tx_buffer.

Drops the (uint8_t*) cast that furi_hal_spi_bus_tx() needed to call
furi_hal_spi_bus_trx_dma() with its own const uint8_t* buffer
parameter, and turns the (uint8_t*)&dma_dummy_u32 cast (the dummy
buffer is itself const uint32_t) into a properly const-preserving
(const uint8_t*) cast.

api_symbols.csv updated to match. Existing in-tree callers
(furi_hal_sd.c) pass non-const pointers and continue to compile
without changes; out-of-tree callers passing const pointers no
longer need to drop qualifiers.

Reported by Copilot review on #4360.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: WonderMr

targets/f7/api_symbols.csv

@@ -1703,7 +1703,7 @@ Function,+,furi_hal_spi_bus_handle_init,void,const FuriHalSpiBusHandle*
 Function,+,furi_hal_spi_bus_init,void,FuriHalSpiBus*
 Function,+,furi_hal_spi_bus_rx,_Bool,"const FuriHalSpiBusHandle*, uint8_t*, size_t, uint32_t"
 Function,+,furi_hal_spi_bus_trx,_Bool,"const FuriHalSpiBusHandle*, const uint8_t*, uint8_t*, size_t, uint32_t"
-Function,+,furi_hal_spi_bus_trx_dma,_Bool,"const FuriHalSpiBusHandle*, uint8_t*, uint8_t*, size_t, uint32_t"
+Function,+,furi_hal_spi_bus_trx_dma,_Bool,"const FuriHalSpiBusHandle*, const uint8_t*, uint8_t*, size_t, uint32_t"
 Function,+,furi_hal_spi_bus_tx,_Bool,"const FuriHalSpiBusHandle*, const uint8_t*, size_t, uint32_t"
 Function,-,furi_hal_spi_config_deinit_early,void,
 Function,-,furi_hal_spi_config_init,void,

targets/f7/furi_hal/furi_hal_spi.c

@@ -113,7 +113,7 @@ bool furi_hal_spi_bus_tx(
 
     if(furi_kernel_is_running()) {
         // Use DMA for TX when scheduler is running, freeing the CPU during transfer
-        bool ret = furi_hal_spi_bus_trx_dma(handle, (uint8_t*)buffer, NULL, size, timeout);
+        bool ret = furi_hal_spi_bus_trx_dma(handle, buffer, NULL, size, timeout);
         LL_SPI_ClearFlag_OVR(handle->bus->spi);
         return ret;
     }
@@ -201,7 +201,7 @@ static void spi_dma_isr(void* context) {
 
 bool furi_hal_spi_bus_trx_dma(
     const FuriHalSpiBusHandle* handle,
-    uint8_t* tx_buffer,
+    const uint8_t* tx_buffer,
     uint8_t* rx_buffer,
     size_t size,
     uint32_t timeout_ms) {
@@ -329,7 +329,7 @@ bool furi_hal_spi_bus_trx_dma(
 
         if(tx_buffer == NULL) {
             // RX mode, use dummy data instead of TX buffer
-            tx_buffer = (uint8_t*)&dma_dummy_u32;
+            tx_buffer = (const uint8_t*)&dma_dummy_u32;
             tx_mem_increase_mode = LL_DMA_MEMORY_NOINCREMENT;
         } else {
             tx_mem_increase_mode = LL_DMA_MEMORY_INCREMENT;

targets/furi_hal_include/furi_hal_spi.h

@@ -118,7 +118,7 @@ bool furi_hal_spi_bus_trx(
  */
 bool furi_hal_spi_bus_trx_dma(
     const FuriHalSpiBusHandle* handle,
-    uint8_t* tx_buffer,
+    const uint8_t* tx_buffer,
     uint8_t* rx_buffer,
     size_t size,
     uint32_t timeout_ms);