docs: update all documentation to reflect 31 completed POSIX tasks

docs: update all documentation to reflect 31 completed POSIX tasks

- POSIX_ROADMAP.md: mark all 31 tasks complete, add remaining gaps in 3 tiers
- README.md: add ASLR, vDSO, futex, guard pages, FAT16, DNS, zero-copy DMA, RTC, MTRR, ld.so stub; update POSIX score to ~90%
- BUILD_GUIDE.md: add ld.so, expanded ulibc details, updated smoke test list
- SUPPLEMENTARY_ANALYSIS.md: fix ~20 stale table entries, update score 70%->90%, rewrite gaps/recommendations/conclusion
- AUDIT_REPORT.md: mark user stack guard pages as fixed, update fix summary
- TESTING_PLAN.md: update current state to reflect all 4 testing layers operational