docs: move the root-level build docs under docs/build/

build-release.md, build-desktop-packages.md and build-windows.md sat at
the docs/ root while docker.md and build-builder-image.md were already in
docs/build/, so "where do build docs live" had two answers. They now have
one.

Referrers updated: README.md, docs-site/SUMMARY.md, and the ../ links
inside the moved files themselves, which each gained a level of depth —
Dockerfile, Dockerfile.arch, packaging/arch/PKGBUILD, CHANGELOG.md,
README.md, src-tauri/src/lib.rs and src/lib/services/webviewAudio.ts.
Every one of those was caught by check-doc-links.sh rather than by
reading, which is the point of having it.

Two referrers are left for their owners: CLAUDE.md line 173 and the
comment at scripts/build-windows-cross.sh line 11.

docs/build/build-windows.md

@@ -0,0 +1,78 @@
+# Windows build
+
+JellyTau targets Linux and Android primarily, but a working Windows build —
+including an **NSIS installer cross-compiled from Linux** — is produced by the
+Docker tooling. It is not yet a first-class release target (no code signing / CI
+job / SMTC lockscreen), but it runs and plays media.
+
+## How playback works on Windows
+
+- **Video** — renders through the webview HTML5 `<video>` element (hls.js) on
+  *every* platform; on Windows that is WebView2 (Chromium/Edge), which plays HLS +
+  h264 fine. No Windows-specific code.
+- **Audio-only (music)** — the native audio backends are libmpv (Linux) and
+  ExoPlayer (Android); neither exists on Windows. Instead
+  `create_player_backend()` in [../src-tauri/src/lib.rs](../../src-tauri/src/lib.rs)
+  uses `WebviewAudioBackend` on non-Linux/non-Android targets: it hands the stream
+  URL to a webview `<audio>` element (see
+  [../src/lib/services/webviewAudio.ts](../../src/lib/services/webviewAudio.ts)),
+  which reports state back through the same `player_report_*` round-trip the video
+  path uses. Pure Rust + Tauri events.
+
+## Cross-compiling from Linux (MSVC + cargo-xwin)
+
+We use the [official Tauri cross-compile path](https://v2.tauri.app/distribute/windows-installer/):
+the **MSVC** target (`x86_64-pc-windows-msvc`) driven by
+[`cargo-xwin`](https://github.com/rust-cross/cargo-xwin), which downloads the MSVC
+CRT / Windows SDK headers and links with `lld`. MSVC is the target Tauri
+officially supports for Windows (mingw/GNU is not), and — unlike GNU — it lets the
+Tauri CLI bundle the **NSIS installer from a Linux host**.
+
+> Why not mingw/GNU? The GNU target *does* link a valid `.exe`, but the Tauri CLI
+> gates `--bundles` by the host OS unless it recognizes a real Windows build.
+> `--runner cargo-xwin --target x86_64-pc-windows-msvc` is what flips it into
+> Windows mode and enables the `nsis`/`msi` bundlers on Linux.
+
+The builder image ([../Dockerfile.builder](../../Dockerfile.builder)) bakes in the
+whole toolchain: the `x86_64-pc-windows-msvc` rust target, `cargo-xwin`, `lld`,
+`llvm`, and `nsis`.
+
+```bash
+bun run docker:build:windows            # NSIS installer + .exe -> ./dist
+WIN_BUNDLES=none bun run docker:build:windows   # exe only, skip bundling
+```
+
+Or directly on a host that has the toolchain:
+
+```bash
+scripts/build-windows-cross.sh                    # nsis installer + exe
+WIN_BUNDLES=none scripts/build-windows-cross.sh   # exe only
+```
+
+Under the hood the build runs:
+
+```bash
+tauri build --runner cargo-xwin --target x86_64-pc-windows-msvc --bundles nsis
+```
+
+Outputs:
+- `.exe` — `src-tauri/target/x86_64-pc-windows-msvc/release/jellytau.exe`
+- NSIS installer — `.../release/bundle/nsis/*-setup.exe`
+
+(both copied to `./dist` when `OUTPUT_DIR` is set).
+
+## Caveats
+
+- **Cross-compilation is a last resort** per Tauri's own docs — it's less tested
+  than building on Windows. If it misbehaves, a `windows-latest` CI job or a
+  Windows VM building natively (`tauri build --bundles nsis`) is the fallback.
+- **Code signing is not wired up** — the installer is unsigned, so Windows
+  SmartScreen will warn on first run.
+
+## Outstanding for a first-class Windows release
+
+1. Gapless/crossfade + SMTC (lockscreen) — currently no-ops in the webview audio
+   path.
+2. Downloaded (`Local` source) file playback needs `convertFileSrc` on the
+   frontend; streaming works today.
+3. Code signing + a Windows packaging CI job.