Cold power-on reset hard-faults before configure() (CFSR IMPREISERR, HFSR 0xC0000000)

[emrainey]

Observed on the Nucleo-H753ZI during SWO bring-up (issue-41 work): a cold POR (power-cycle / unplug the USB) consistently hard-faults in on_startup(), before cortex::system::configure() runs.

Symptoms

• CFSR: IMPREISERR (imprecise bus error) on the PRECISERR bit.
• HFSR: 0xC0000000 (FORCED + VECTBL).
• Faulting frame in on_startup() at boot, prior to configuration; reloading via the debugger (reset + run) does not reproduce.
• Software reset (SYSRESETREQ) does not reproduce; only a cold POR does.

Notes

• D3DBGCKEN (DBGMCU_CR bit 22) cannot be set pre-boot, so the SWO-on-boot path could not be inspected for this fault.
• Deferred from Enable Serial Wire Output (SWO) for debug tracing #41; tracked here so the SWO work can proceed independently.

Reproduction

1. Power the Nucleo-H753ZI fully off, then on.
2. Attach the debugger (J-Link) and inspect CFSR/HFSR.
3. Fault is present before main/configure runs.