docs: update all documentation with recent kernel/userspace/test changes

docs: update all documentation with recent kernel/userspace/test changes

README.md:
- Host tests: 69 → 212 (test_utils 63, test_security 38, host_utils 111)
- Battery checks: 33 → 152 (120 smoke + SMP=1/2 + multi-disk + VFS + ping)
- GDB checks: 6 → 10 (PID state, scheduler bitmap, mount count, frame refcount)
- W^X description: updated for elf32_reprotect_segments() + boundary page handling
- tests/ directory description updated

TESTING_PLAN.md:
- All test counts updated
- Pure function list expanded with tar_parse_octal, mount prefix/normalize,
  vfs_check_permission, elf32_validate, signal mask logic, parse_symbolic
- Makefile targets updated

SYSCALL_TEST_COVERAGE.md:
- Date updated to 2026-04-27
- Battery: 33 → 152, Host: 69 → 212

POSIX_ROADMAP.md:
- Added host unit tests (212), test battery (152), GDB checks (10) rows
- Host utility test harness: 68 → 111

AUDIT_REPORT.md:
- Added post-audit fix table: ELF W^X reprotect, vfs_lookup_initrd,
  heap corruption handling, SHM UAF, procfs UAF, ext2 partial inode,
  rq_remove_if_queued, execve_copy_user_str, frame refcount overflow

TIER6_PLAN.md / FULL_POSIX_AUDIT.md:
- Test counts and commit references updated