vgsp-gr: keep the validated fixed CB across launches and map each VTG stream contiguously

- Remember the last fixed-CB page that resolved through a SYS source and
  passed the matrix gate; a later resolution miss (or a method-page answer)
  reuses it instead of zeroing the seed page with the synthetic identity.
  Only a boot with no validated read ever uses the identity fallback. Do not
  skip the write on an unchanged hash: the seed page lives in the client's
  screen allocation and the guest can rewrite it between launches.
- Treat the CALL_MME_MACRO(2) vertex stream as one range: map [base,end) into
  a fresh contiguous fetch line and rewrite both ends from it, preserving the
  client's stream length across 4 KB boundaries. Unresolvable pages map the
  module zero page (a zero vertex, never a fault). The generic per-word pass
  remains the fallback.
- Diagnostics: cb-copy logs the first 8 seed-page words and the cached flag;
  MME-VSTREAM-LINE dumps the resolved base page (client VA, HPA, page count,
  first 8 words).
- Evidence: r1494 (poison 11/17 -> 1/16, real vertex floats at the stream
  bases, CB cache verified live with src=vaswalk -> src=cached-real) and
  r1495/r1496 (the active draw's vertex/index page is an all-zero vidmem
  shadow page while the data exists in guest sysmem - the new wall).

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