docs: add Tier 6 implementation plan — SMP, multi-arch, Rump, HDA, Bash/Busybox

docs: add Tier 6 implementation plan — SMP, multi-arch, Rump, HDA, Bash/Busybox

Comprehensive plan for remaining large architectural features:
- 6A: Full SMP scheduling (per-CPU current, AP activation, load balancing)
- 6B: Multi-arch subsystems (ARM64/RISC-V/MIPS PMM+VMM+scheduler)
- 6C: Rump Kernel integration (USB, production drivers)
- 6D: Intel HDA audio driver
- 6E: USTAR initrd format
- 6F: Minor POSIX gaps (madvise, mntent, utmp)
- 6G: Bash & Busybox ports (all blockers resolved)

Recommended order: Bash/Busybox → minor gaps → SMP → Rump → USTAR → HDA → multi-arch