Add documentation

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

playback/engine-mpv/build.gradle.kts

@@ -1,4 +1,4 @@
-// The desktop's playback engine (LINUX_PLAN.md §2.3): libmpv through JNA, video drawn offscreen and shown in Compose.
+// The desktop's playback engine (docs/desktop.md): libmpv through JNA, video drawn offscreen and shown in Compose.
 plugins {
     alias(libs.plugins.kotlin.jvm)
     alias(libs.plugins.kotlin.compose)

playback/engine-mpv/src/main/kotlin/ru/stenfredd/sleepyowl/playback/mpv/MpvEngine.kt

@@ -28,7 +28,7 @@ import kotlin.concurrent.thread
 import kotlin.math.roundToInt
 
 /**
- * libmpv engine of the desktop (LINUX_PLAN.md §2.3). One mpv core per playback, without a window of its own
+ * libmpv engine of the desktop (docs/desktop.md). One mpv core per playback, without a window of its own
  * (`vo=libmpv`): the picture comes out of [video] and is shown by [MpvVideoSurface].
  *
  * Commands go to mpv asynchronously, so the UI thread never waits on the core; mpv's events and observed properties are

playback/engine-mpv/src/main/kotlin/ru/stenfredd/sleepyowl/playback/mpv/MpvEngineProvider.kt

@@ -26,7 +26,7 @@ val mpvEngineModule = module {
 
 /**
  * mpv plays everything ffmpeg demuxes and decodes, in hardware where the GPU can (NVDEC, VAAPI), otherwise on the CPU,
- * which a PC can afford. HDR and Dolby Vision are tone-mapped to SDR (the desktop has no HDR output, LINUX_PLAN.md §0).
+ * which a PC can afford. HDR and Dolby Vision are tone-mapped to SDR (the desktop has no HDR output, see docs/desktop.md).
  */
 class MpvEngineProvider : EngineProvider {
     override val id = ID

playback/engine-mpv/src/main/kotlin/ru/stenfredd/sleepyowl/playback/mpv/MpvVideo.kt

@@ -21,7 +21,7 @@ import kotlin.math.min
 import kotlin.math.roundToInt
 
 /**
- * mpv's picture, variant B of LINUX_PLAN.md §2.3: mpv draws in its own invisible OpenGL context (EGL pbuffer) on its
+ * mpv's picture, variant B of docs/desktop.md: mpv draws in its own invisible OpenGL context (EGL pbuffer) on its
  * own thread, into an FBO; the frame is read back through a persistently mapped PBO into memory that Compose shows
  * ([MpvVideoSurface]). mpv knows nothing about the window, so the window can be rebuilt freely.
  *