feat: Rump Kernel hypercall scaffold + roadmap documentation

feat: Rump Kernel hypercall scaffold + roadmap documentation

- docs/RUMP_KERNEL_ROADMAP.md: full integration plan with 4 phases
  (filesystem → network → USB → audio), API mapping table,
  prerequisites checklist, and build integration notes
- src/rump/rumpuser_adros.c: Phase 1+3 hypercall scaffold implementing:
  rumpuser_init, rumpuser_malloc/free, rumpuser_putchar/dprintf,
  rumpuser_exit, rumpuser_getparam, rumpuser_getrandom,
  rumpuser_clock_gettime/sleep
- Phase 2 (threads/sync) and Phase 4 (file/block I/O) are TODO stubs