docs: update all documentation with current test coverage and POSIX gap analysis

docs: update all documentation with current test coverage and POSIX gap analysis

- Update test counts across all docs: 120 smoke tests, 33 battery, 69 host
- Update syscall count: 141 syscalls, 124/141 tested (87.9% coverage)
- Add syscall test coverage section listing 17 untested syscalls
- Add comprehensive POSIX gaps section for 100% compliance
- Categories: process/credentials, filesystem, signals, POSIX timers,
  IPC, memory, network, threads
- Mark madvise and execveat as implemented in FULL_POSIX_AUDIT.md