vgsp: per-channel MME pack completion accounting + rearm release + context-reset invalidation (eng40, validation pending)

After the multi-part beef pack completes (2799/2799; keep_gen=1 works), the
prelude generation drifts and the pack branch refuses re-emission (poff==maxa),
so calls stay held forever (pcv=0, shadow draws 0, wrote=0). This adds:
(1) per-channel contiguous-load accounting + pack_done_h so the completion is
remembered for the loaded channel; (2) an MME-PACK-REARM call release when a
completed pack's channel launches again; (3) gsp_phys_gr_mme_chan_ctx_reset(h)
invalidation on context wipes/channel re-registers (the client recycles the
beef channel between pushes, c496), shown firing safely. Validation: the fix
was not yet observed completing on a green window with a same-incarnation draw;
committed for reproducibility per house style.

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