gsp-phys: stage-3b boot layout plus a hazard gate for the claim path

Stage 3b allocates the three coherent boot-argument pages and fills the
256-byte WPR metadata from the computed Turing FB layout; no device
registers are written (uncommitted work from the previous session,
committed as-is behind the new gate).

The physical claim unbinds the distribution NVIDIA driver; on this host
that power-gates the dGPU board and the re-bind can leave the GPU
unresponsive and hard-freeze the system (observed twice, 2026-09-30, no
crash record). The claim now refuses unless the new gsp_phys_hazard_ack
module parameter is also set, so a stale command line cannot re-trigger
it. See the hazard note at the top of gsp_phys_gpu.c.

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