add HYPRLAND.md with integration reference and ricer examples

HYPRLAND.md

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+# Hyprland integration
+
+I daily-drive booru-viewer on Hyprland and I've baked in my own opinions
+on how the app should behave there. By default, a handful of `hyprctl`
+dispatches run at runtime to:
+
+- Restore the main window's last floating mode + dimensions on launch
+- Restore the popout's position, center-pin it around its content during
+  navigation, and suppress F11 / fullscreen-transition flicker
+- "Prime" Hyprland's per-window floating cache at startup so a mid-session
+  toggle to floating uses your saved dimensions
+- Lock the popout's aspect ratio to its content so you can't accidentally
+  stretch mpv playback by dragging the popout corner
+
+## Opting out
+
+If you're a ricer with your own `windowrule`s targeting
+`class:^(booru-viewer)$` and you'd rather the app keep its hands off your
+setup, there are two independent opt-out env vars:
+
+- **`BOORU_VIEWER_NO_HYPR_RULES=1`** — disables every in-code hyprctl
+  dispatch *except* the popout's `keep_aspect_ratio` lock. Use this if
+  you want app-side window management out of the way but you still want
+  the popout to size itself to its content.
+- **`BOORU_VIEWER_NO_POPOUT_ASPECT_LOCK=1`** — independently disables
+  the popout's aspect ratio enforcement. Useful if you want to drag the
+  popout to whatever shape you like (square, panoramic, monitor-aspect,
+  whatever) and accept that mpv playback will letterbox or stretch to
+  match.
+
+For the full hands-off experience, set both:
+
+```ini
+[Desktop Entry]
+Name=booru-viewer
+Exec=env BOORU_VIEWER_NO_HYPR_RULES=1 BOORU_VIEWER_NO_POPOUT_ASPECT_LOCK=1 /path/to/booru-viewer/.venv/bin/booru-viewer
+Icon=/path/to/booru-viewer/icon.png
+Type=Application
+Categories=Graphics;
+```
+
+Or for one-off launches from a shell:
+
+```bash
+BOORU_VIEWER_NO_HYPR_RULES=1 booru-viewer
+```
+
+## Writing your own rules
+
+If you're running with `BOORU_VIEWER_NO_HYPR_RULES=1` (or layering rules
+on top of the defaults), here's the reference.
+
+### Window identity
+
+- Main window — class `booru-viewer`
+- Popout — class `booru-viewer`, title `booru-viewer — Popout`
+
+> ⚠ The popout title uses an em dash (`—`, U+2014), not a hyphen. A rule
+> like `match:title = ^booru-viewer - Popout$` will silently match
+> nothing. Either paste the em dash verbatim or match the tail:
+> `match:title = Popout$`.
+
+### Example rules
+
+```ini
+# Float the popout with aspect-locked resize and no animation flicker
+windowrule {
+    match:class = ^(booru-viewer)$
+    match:title = Popout$
+    float = yes
+    keep_aspect_ratio = on
+    no_anim = on
+}
+
+# Per-window scroll factor if your global is too aggressive
+windowrule {
+    match:class = ^(booru-viewer)$
+    match:title = Popout$
+    scroll_mouse = 0.65
+}
+```
+
+### What the env vars actually disable
+
+`BOORU_VIEWER_NO_HYPR_RULES=1` suppresses the in-code calls to:
+
+- `dispatch resizeactive` / `moveactive` batches that restore saved
+  popout geometry
+- `dispatch togglefloating` on the main window at launch
+- `dispatch setprop address:<addr> no_anim 1` applied during popout
+  transitions
+- The startup "prime" sequence that warms Hyprland's per-window
+  floating cache
+
+`BOORU_VIEWER_NO_POPOUT_ASPECT_LOCK=1` suppresses only
+`dispatch setprop address:<addr> keep_aspect_ratio 1` on the popout.
+Everything else still runs.
+
+Read-only queries (`hyprctl clients -j`, `hyprctl monitors -j`) always
+run regardless — the app needs them to know where it is.
+
+### Hyprland requirements
+
+The `keep_aspect_ratio` windowrule and `dispatch setprop
+keep_aspect_ratio` both require a recent Hyprland. On older builds the
+aspect lock is silently a no-op.