docs: update README, BUILD_GUIDE, POSIX_ROADMAP, TESTING_PLAN for current state

docs: update README, BUILD_GUIDE, POSIX_ROADMAP, TESTING_PLAN for current state

- README: buddy allocator heap, ICMP ping, IOAPIC level-triggered,
  multi-drive ATA, kernel cmdline, kconsole, NO_SYS=0, 20 smoke tests,
  16-check test battery, ~103K LOC across 255 commits, ~95% POSIX coverage
- BUILD_GUIDE: networking, multi-disk QEMU, root= cmdline param,
  test-battery target, 20 smoke checks
- POSIX_ROADMAP: FAT12/16/32 full RW (was FAT16 RO), ext2 full RW (was
  not implemented), NO_SYS=0 threaded mode, ICMP ping, multi-drive ATA,
  kconsole, buddy allocator, 48 total features (31+17), updated remaining
  work tiers
- TESTING_PLAN: 20 smoke checks, test-battery description, updated
  Makefile targets