IP22 (R4400 Indigo2) with any IMPACT board: PROM panics (UTLB miss at 0x10c) before IRIX #270

An Indigo2 IP22 (R4400) with any IMPACT board stops in the PROM on main (9f3a4da), before IRIX loads:

NVRAM checksum is incorrect: reinitializing.

Exception: <vector=UTLB Miss>
Status register: 0x30004802<CU1,CU0,IM7,IM4,IPL=???,MODE=KERNEL,EXL>
Cause register: 0x8<CE=0,EXC=RMISS>
Exception PC: 0x9fc36988, Exception RA: 0x9fc16fa0
exception, bad address: 0x10c
...
PANIC: Unexpected exception

• Config: machine.profile = "indigo2_ip22", cpu = "r4400", banks = [128, 128, 128, 0], prom = "" (the embedded IP22 PROM), graphics.board = solidimpact, highimpact or maximpact. All three stop at the same PC and RA.
• The same PROM, NVRAM and disk boot fine with newport, xz and extreme, so it is the IMPACT path on IP22, not the NVRAM file: the "checksum is incorrect" line appears only with IMPACT fitted, with either of two NVRAM files that boot the other boards.
• IP28 with Solid, High and Max IMPACT boots fine on the same build.
• This configuration booted on 2026-10-01 (a clean IRIX 6.5.22m install was done on IP22 + Solid IMPACT then), so something since then broke it; I haven't bisected yet.

Found while regression-testing #261 across every board on every machine; that PR does not change it (identical on main).

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