feat(windows): a Windows target for ludicc, and the compiler builds itself there

--target <triple> (default: the host, from OS=Windows_NT) selects the Windows
runtime. emit_win.ludic defines the POSIX names the backend already calls over
the UCRT and Win32 in IR, so rename replaces an existing file, ftell is 64-bit,
and fopen is binary. Known folders follow %APPDATA% / %LOCALAPPDATA% / %TEMP%,
and the driver speaks cmd.exe, writes .exe outputs and finds LLVM's clang.

Verified: macOS three-stage fixpoint, ludic-dev test 135/135, and on Windows
the Mac-emitted Windows IR and the Windows-built compiler's IR are identical
through two generations.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

changes/windows-target.md

@@ -0,0 +1,20 @@
+bump: minor
+type: feat
+**Windows target** — `ludicc` builds and runs headless programs on Windows, and builds itself there.
+
+- **`--target <triple>`** — a triple naming `windows` selects the Windows runtime; without
+  the flag the target is the host, read at run time from `OS=Windows_NT`. The IR still
+  carries no triple, so clang assembles it for the machine it runs on.
+- **The libc surface, in IR** — on Windows the header emits `emit_win.ludic`, which defines
+  the POSIX names the backend calls (`fopen`, `ftell`, `rename`, `opendir`, `mmap`,
+  `fmemopen`, `uname`, …) over the UCRT and Win32. Three of them fixed silent breakage
+  rather than link errors: `rename` onto an existing file (every `Fs.write_text` after the
+  first), a 32-bit `ftell`, and text-mode `fopen` rewriting `\n` as `\r\n`.
+- **Known folders** — `Os.save_dir` / `config_dir` are `%APPDATA%\<app>`, `cache_dir` is
+  `%LOCALAPPDATA%\<app>`, `temp_dir` is `%TEMP%`, all with forward slashes; a bundle's
+  `home` line points at `%APPDATA%\<name>`.
+- **The driver** — finds `C:\Program Files\LLVM\bin\clang.exe` when clang is not on
+  `%PATH%`, writes `.exe` outputs, speaks cmd.exe for its directory and cleanup commands,
+  and reads `argv[0]`, `%PATH%` and `$LUDIC_HOME` with either separator.
+- **Not yet** — a window, `Gl.*`, `Audio.*` and `Http.*` are still macOS platform layers;
+  a Windows build that needs one stops with a message saying so.