Specify a Windows installer built by the Linux CI

The tree is closer to Windows than a Linux-only project usually is:
every image library, the TLS stack and the inference engine are pure
Rust, and dr-plat already keeps the Linux-only code behind cfgs with a
loud fallback where none exists for another platform. What remains is
a short list above dr-plat — five XDG path lookups, an xdg-open, the
secret store's third implementation, the models' lookup beside the
executable — and none of it touches core, which is the NFR-PORT-3 test
this would be the first real run of.

docs/windows.md decides the GNU target over MSVC-via-xwin, Vulkan only
as on every other platform, a per-user NSIS installer that leaves the
library alone on uninstall, and a CI leg in the shape of the Android
one. It is explicit about what a runner with no Windows can verify —
that it links, is PE32+, starts under Wine and installs under Wine —
and what it cannot, which is everything involving a real GPU driver.
Three FR-PLAT-WIN requirements and a channel row record the decisions;
the ordering puts a first cross-compile on the developer machine before
any container exists, because the list of cfg gaps is a reading of the
source and the compiler's list will be longer.

docs/requirements.md

@@ -1009,6 +1009,24 @@ protocol is unavailable, FR-DSP-8's stated fallback applies.
 portals and credential storage uses the Secret Service portal, both verified to satisfy FR-NC-2 and
 FR-CAT-1 within the sandbox.
 
+#### Windows
+
+Specified in [windows.md](windows.md); a stated channel under NFR-COMPAT-2, not a v1 one.
+
+**FR-PLAT-WIN-1 — Known folders.** Configuration under `%APPDATA%\darkroom`; data, cache and
+state under `%LOCALAPPDATA%\darkroom`. Nothing under the profile root and nothing relative to the
+working directory. The layout beneath those roots is the same as under XDG, so a library directory
+moves between platforms unchanged.
+
+**FR-PLAT-WIN-2 — Installer.** A per-user installer that needs no elevation, registers an
+uninstaller, and whose uninstaller removes what the installer wrote and nothing the application
+wrote. Models are installed beside the executable and found there last, after the user's own
+directories.
+
+**FR-PLAT-WIN-3 — Built from Linux.** The Windows binary and its installer are produced by the
+Linux CI from the same commit as every other channel, with no Windows machine in the build.
+Verification on Windows is a release step, recorded per release, not a build step.
+
 ### 3.9 Culling
 
 Per D11 this is the product's primary differentiator, not an incidental capability.