v0.5.5: visible boot progress + panic-on-screen for real hardware

Real hardware has no serial console: a pre-shell panic was a silent
blank screen (the panic handler only wrote to serial, then cli;hlt).
v0.5.4's shadow change made this window reachable — if the fb-aware
page allocation or the heap fallback failed on hardware, the kernel
halted before the shell banner with zero screen output.

- gop::boot_marker(): every boot stage now draws straight to the
  framebuffer (row 0, white-on-black).  The last marker visible before
  a hang identifies where the kernel died.
- Panic handler: pre-shell panics now draw a white-on-red
  '*** PANIC *** <msg> at file:line' band via gop::dump_lines.
- init_shadow(): heap fallback is panic-proof (try_reserve_exact +
  resize instead of vec![0; words] which aborts on OOM); page-alloc
  result defensively re-checked against the fb region; any failure
  degrades to no-shadow mode with a visible warning band.
- Verified: full chain (rsboot -> kernel) boots with markers, shell
  banner renders, all 5 test suites pass.

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