Fix STM32 boards crashing instead of running out of memory

[mikeysklar]

What

Keep the heap out of the stack's RAM on 18 STM32 boards.

Why

A big allocation could land on the stack and crash the board instead of raising MemoryError.

The largest free block on the Feather STM32F405 shrinks by about 24 KB.

Code that fit only by overlapping the stack now raises MemoryError.

Tested
Board	Feather STM32F405 Express
Host	Ubuntu 24.04.5
Firmware	10.3.1, main 11.0.0-alpha.1-86, this PR
Not tested	H7, F7, L4, thunderpack
Builds	feather_stm32f405_express, nucleo_h743zi_2, nucleo_f746zg

Probe, 10 trials each, power cycle between	10.3.1	main	this PR
Fill largest block, recurse to stack limit	10/10 STACK_OVERFLOW	10/10 STACK_OVERFLOW	10/10 clean
Same, 32 KB block		1/1 STACK_OVERFLOW	10/10 clean

Probe code and output

import gc
gc.collect()
n = 90112
b = None
while b is None:
    try:
        b = bytearray(b'\xa5') * n
    except MemoryError:
        n -= 1024
def r(d):
    return r(d + 1)
try:
    r(0)
except RuntimeError:
    pass
pat = bytearray(b'\xa5') * 1024
bad = 0
for i in range(0, n, 1024):
    if b[i:i + 1024] != pat[:len(b[i:i + 1024])]:
        bad += 1
print('RESULT', 'CORRUPT' if bad else 'CLEAN', n, bad)

main:    board resets; after reconnect, supervisor.runtime.safe_mode_reason is supervisor.SafeModeReason.STACK_OVERFLOW
this PR: RESULT CLEAN 63488 0

AI assistance: written with Claude Code. I reviewed the change and the trial results.

🤖 Generated with Claude Code

[tannewt]

Why not change the linker script?

[tannewt]

Thank you!