v0.5.5: self-diagnosing boot — vt refuses 0x0 geometry, panic band shows last stage

The user's real-hardware boot showed 'PANIC: index out of bounds (len 0,
index 0) at vt.rs:146' — the vt grid was created empty because the
framebuffer geometry was 0x0 at vt::init (GOP missing or the active
mode reporting zero resolution).  The v0.5.4 blank screen was the same
panic, previously invisible (no screen dump in the panic handler).

- vt::init: rows/cols == 0 now halts with a visible white-on-red
  'FATAL: no framebuffer geometry — GOP missing or active mode is 0x0'
  band instead of building an empty grid that panics at the first
  cell write.
- boot_marker records the last boot stage; the panic band appends it
  as a 4th line so a crash shows exactly how far boot got.
- The gop+vt marker now carries the measured geometry ('rsnix: gop
  800x600 vt 100x37 ok') so a good boot is self-confirming on the
  screen.
- All 5 test suites pass; build/clippy clean.

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