docs: update README, POSIX_ROADMAP, TESTING_PLAN, BUILD_GUIDE for all 66 features

docs: update README, POSIX_ROADMAP, TESTING_PLAN, BUILD_GUIDE for all 66 features

README.md:
- ARM64/RISC-V now listed as bootable on QEMU virt (not just build infra)
- Added SMAP, per-CPU runqueues, posix_spawn, interval timers, IPv6,
  DHCP, getaddrinfo, virtio-blk, dlopen/dlsym, sigqueue, waitid,
  POSIX mq_*/sem_*, pipe capacity fcntl, select/poll for files
- Running section now includes ARM64 and RISC-V commands
- Directory structure includes src/arch/arm/ and src/arch/riscv/
- Status updated to 66 total features, ~98% POSIX coverage

POSIX_ROADMAP.md:
- All 18 new features marked [x] in their respective tables
- Progress list extended to items 49-66
- Remaining Work section replaced: all gaps resolved, future
  enhancements listed (epoll, inotify, sendmsg/recvmsg, aio_*)

TESTING_PLAN.md:
- Added multi-arch build verification line
- Added qemu-system-aarch64 and qemu-system-riscv64 to tools table
- Added make run-arm / make run-riscv to Makefile targets

BUILD_GUIDE.md:
- Updated feature summary paragraph
- Fixed ld.so description (full relocation, not stub)
- ARM64 section: added make run-arm shortcut and expected output
- RISC-V section: fixed QEMU command (-bios none), added expected output
- Renumbered Common Troubleshooting to section 6