f469-disco: add missing simulator stubs (LED, SDCard, hard_reset)

[Amperstrand]

Summary

f469-disco/libs/unix/pyb.py is missing several class/function stubs that are available on hardware but not in the simulator:

1. LED — used by some firmware code for status indication
2. SDCard — needed if any code references SD card operations
3. hard_reset() — pyb.hard_reset() is called by HIL test code (TEST_RESET command) but not defined in simulator

Proposed additions to libs/unix/pyb.py

class LED:
    def __init__(self, id):
        self.id = id
    def on(self):
        pass
    def off(self):
        pass
    def toggle(self):
        pass

class SDCard:
    def __init__(self, *args, **kwargs):
        pass
    def readblocks(self, *args, **kwargs):
        pass
    def writeblocks(self, *args, **kwargs):
        pass
    def ioctl(self, *args, **kwargs):
        return 0

def hard_reset():
    import sys
    sys.exit()

Impact

Without hard_reset(), importing hil.py on the simulator fails because HILCommandHandler._process_line("TEST_RESET") calls pyb.hard_reset().

Status

• Changes were made locally but reverted to keep submodule clean
• Should be submitted as PR to diybitcoinhardware/f469-disco