ports/stm32: Use dynamic heap with proper .noinit section handling.

Replace the 80KB static heap with dynamic heap allocation by properly
handling the .noinit section in the linker script. Zephyr kernel stacks
are placed in .noinit, and the heap now starts after this section.

This reduces BSS from 179KB to 97KB when threading is enabled, and has
zero overhead when threading is disabled (.noinit is empty).

Changes:
- Add .noinit section to common_bss_heap_stack.ld with _enoinit symbol
- Update stm32f429.ld to use _enoinit for _heap_start
- Remove static 80KB heap array from main.c, use MICROPY_HEAP_START/END

Author: pi-anl

ports/stm32/boards/common_bss_heap_stack.ld

@@ -11,6 +11,15 @@
     _ebss = .;
 } >RAM
 
+/* Non-initialized data section (Zephyr kernel stacks when threading enabled) */
+.noinit (NOLOAD) :
+{
+    . = ALIGN(4);
+    *(.noinit*)
+    . = ALIGN(4);
+    _enoinit = .;
+} >RAM
+
 /* This is to define the start of the heap, and make sure there is a minimum size */
 .heap :
 {

ports/stm32/boards/stm32f429.ld

@@ -26,7 +26,7 @@ _sstack = _estack - 16K; /* tunable */
 /* RAM extents for the garbage collector */
 _ram_start = ORIGIN(RAM);
 _ram_end = ORIGIN(RAM) + LENGTH(RAM);
-_heap_start = _ebss; /* heap starts just after statically allocated memory */
+_heap_start = _enoinit; /* heap starts after .bss and .noinit sections */
 _heap_end = _sstack;
 
 /* Filesystem cache in RAM, and storage in flash */

ports/stm32/main.c

@@ -312,15 +312,6 @@ static bool init_sdcard_fs(void) {
 #endif
 
 #if MICROPY_ZEPHYR_THREADING
-// Static heap for Zephyr threading builds
-// Thread stacks are now dynamically allocated from this heap (not pre-allocated in BSS)
-// Heap size increased to 80KB to accommodate both Python objects and thread stacks
-// NUCLEO_F429ZI has 192KB RAM total.
-// Previous: 42KB heap + 40KB pre-allocated stacks in BSS = 82KB for MicroPython use
-// Current: 80KB heap with dynamic stack allocation = same effective capacity
-#define MICROPY_HEAP_SIZE (80 * 1024)
-static char heap[MICROPY_HEAP_SIZE];
-
 // Zephyr threading entry point - called by Zephyr kernel after z_cstart()
 // This function runs in z_main_thread context after kernel initialization
 void micropython_main_thread_entry(void *p1, void *p2, void *p3) {
@@ -362,8 +353,8 @@ void micropython_main_thread_entry(void *p1, void *p2, void *p3) {
     size_t stack_size = z_main_thread.stack_info.size;
     mp_cstack_init_with_top(stack_top, stack_size);
 
-    // GC init with static heap (avoids overlap with thread stacks in .noinit section)
-    gc_init(heap, heap + sizeof(heap));
+    // GC init (linker script places .noinit after .bss, so _heap_start is safe)
+    gc_init(MICROPY_HEAP_START, MICROPY_HEAP_END);
 
     // Threading init - Phase 2 (allocate main thread on GC heap)
     // Requires GC to be initialized for m_new_obj() heap allocation