docs: update README, BUILD_GUIDE, TESTING_PLAN for MIPS + expanded tests

docs: update README, BUILD_GUIDE, TESTING_PLAN for MIPS + expanded tests

- README.md: MIPS32 now boots on QEMU Malta, added run-mips instructions,
  updated test counts (41 smoke, 19 host), added src/arch/mips/ to directory
- BUILD_GUIDE.md: added section 6 (MIPS32 build & run), renumbered troubleshooting
- TESTING_PLAN.md: updated smoke test count to 41, added 6 new test descriptions,
  added qemu-system-mipsel to tools table, added make run-mips target