build: add Windows build support via zig cc and GNU objcopy

[coff33ninja]

What

Adds a Windows build path alongside the macOS Makefile. These three scripts make the tree build on a stock Windows box with Zig's bundled clang (zig cc), NASM, QEMU and GNU binutils from MSYS2:

• scripts/ensure-toolchain.ps1 - provisions zig, nasm, qemu, python and the MSYS2 mingw-w64-x86_64-binutils package; records resolved paths in a gitignored scripts/.toolchain.json.
• scripts/build.ps1 - compiles the same 211 objects as the Makefile with the same flags (minus the GCC-only ones), links with lld, then re-labels the ELF64 image as an ELF32 container with objcopy -O elf32-i386 - the exact command the Makefile's $(OBJCOPY) step runs. Verified end-to-end: kernel.bin boots, formats disk.img FAT32, and persists across reboots.
• scripts/run.ps1 - boots kernel.bin in QEMU with the same netdev/e1000/IDE/fw_cfg flags and the same .env API-key rules as the Makefile.

README: new "On Windows" section, plus a footer crediting NO_OS - the ELF-reframing ADR there is where the objcopy trick came from.

Follow-up: the QEMU boot-assertion suite also runs on Windows

• scripts/test-qemu.ps1 - entry point mirroring the Makefile's test-qemu recipe: tests/qemu/lint_printf.py (the lib/kfmt.c formatter-grammar check), then all six boot-assertion cases.
• tests/qemu/harness.py - ported to Windows Python: signal registration and process reaping (no SIGKILL or process groups), and the interactive REPL case speaks to the VM over a loopback TCP serial port where POSIX uses an AF_UNIX socket. The per-case build hook (FABLEOS_TEST_MAKE) points at build.ps1 so the build-cflags: AC'97 bring-up case works with no make on the box.
• scripts/build.ps1 - objects are now linked in the Makefile's source order, not directory order, because the self-registering driver table initializes in link order and the boot cases pin that order.

Verified on Windows: lint_printf CLEAN, all 6 cases pass (the AC'97 variant build restores the tree afterwards; kernel.bin is byte-identical).

Notes for the maintainer

• QEMU's multiboot loader only accepts ELF32; zig's bundled llvm-objcopy can only emit flat binary, so the one missing piece was a real GNU objcopy. The reframing follows coff33ninja/NO_OS (linked in the README).
• -fno-sanitize=undefined is required because zig's compiler_rt pulls in __ubsan_handle_* on freestanding builds.
• The README footer names the fork location - happy to trim it from the PR if you'd rather not carry it upstream.