Add real desktop icons plus right-click desktop/icon context menus

Closes "right-click on bare desktop" - previously a true no-op, nothing
rendered above the wallpaper at all. Requested directly: a real desktop
"just like windows does" - Home/Computer/Trash plus one icon per real
~/Desktop entry, individually draggable with persisted grid positions,
double-click to open, right-click menus (per-icon "Open" plus "Set as
Wallpaper" for image files when general.wallpaper_command is set; bare
desktop "New Folder"/"Refresh").

Architecture mirrors the existing context_menu.rs/snap_flyout.rs
"compositor-owned floating UI" pattern: desktop_icons.rs (data model, grid
layout, filesystem scan, hit-test), desktop_icons_state.rs (JSON
persistence, same shape as monitor_layout.rs), desktop_menu.rs (the new
right-click menu, reusing decoration::render_context_menu's existing
rasterizer), state/desktop_icons.rs (CompState glue: rescan/select/drag/
open/persist), and a new decoration::render_desktop_icon rasterizer --
hand-drawn glyphs, since no PNG/SVG decoding capability exists anywhere in
this workspace. Wired into both render loops (udev and winit) above the
wallpaper and below every window, and into input/pointer.rs's button/
motion handlers for selection, drag, double-click, and both menus.

Four new config keys: general.desktop_icons (default true - a directly
requested, purely visual feature, unlike the opt-in-while-experimental
general.gpu), general.file_manager, general.desktop_icon_single_click,
general.wallpaper_command (all default off/empty).

Deliberately out of scope for this pass, stated up front: move-to-trash
and "Empty Trash" (destructive, no confirmation-dialog primitive to gate
them on yet), filesystem watching, multi-select, per-mimetype icon art,
icons on any monitor but the primary one.

124 wayland-crate tests (up from 106), full workspace build and clippy
clean.

1 files changed, 43 insertions, 0 deletions

diff --git a/docs/DEFAULTS.md b/docs/DEFAULTS.md
index 7e712c5..5f99446 100644
--- a/docs/DEFAULTS.md
+++ b/docs/DEFAULTS.md
@@ -17,6 +17,10 @@ srd.set("general.rounded_corners", true)               -- Default: true on GLES/
 srd.set("general.focus_follows_mouse", false)          -- Default: false - hover a window to focus it, no click needed
 srd.set("general.auto_raise", false)                   -- Default: false - also raise on hover-focus, not just focus
 srd.set("general.gpu", false)                          -- Default: false - udev backend only, see "GPU rendering" below
+srd.set("general.desktop_icons", true)                 -- Default: true - see "Desktop icons" below
+srd.set("general.file_manager", "")                    -- Default: "" - empty means dispatch via `xdg-open`
+srd.set("general.desktop_icon_single_click", false)    -- Default: false - double-click opens an icon
+srd.set("general.wallpaper_command", "")               -- Default: "" - empty hides "Set as Wallpaper" entirely
 ```
 `general.smart_placement`/`general.border_width` are not listed: neither
 is implemented - new-window placement always uses smart placement
@@ -54,6 +58,45 @@ separate, lower-level override for testing without touching config --
 either it or `general.gpu` being set is enough to
 attempt GPU rendering.
 
+#### Desktop icons (`general.desktop_icons`)
+
+Real, individually-draggable desktop icons rendered above the wallpaper
+and below every window, on the primary monitor only: fixed **Home**
+(`$HOME`), **Computer** (`/`), and **Trash** (`~/.local/share/Trash/files`,
+or `$XDG_DATA_HOME/Trash/files` if set) icons, plus one per real,
+non-hidden entry of `~/Desktop` (created if it doesn't exist yet). On by
+default - unlike `general.gpu`, this is a purely visual, directly
+requested feature with no hardware-support question to hedge against.
+
+Hand-drawn glyphs, not real icon-theme artwork - no PNG/SVG decoding
+capability exists anywhere in this codebase, so folder/computer/trash/file
+icons are simple flat shapes, the same technique the titlebar's own
+buttons use.
+
+Double-click (or a single click when `general.desktop_icon_single_click`
+is `true`) opens an icon: `$general.file_manager <path>` if that key is
+set, otherwise `xdg-open <path>`. Dragging an icon snaps it to the nearest
+free grid cell on release and persists that cell to
+`$XDG_STATE_HOME/srd/desktop-icons.json` (else `~/.local/state/srd/...`) --
+only icons the user has actually moved get an entry there; everything
+else keeps recomputing its default slot on every rescan.
+
+Right-click an icon: "Open", plus "Set as Wallpaper" when it's an image
+file (`.png`/`.jpg`/`.jpeg`/`.webp`/`.bmp`/`.gif`) and `general.
+wallpaper_command` is set - shells out to `<wallpaper_command> <path>`.
+srdwm never draws the wallpaper itself (an external layer-shell client's
+job - `swww`, `awww`, or similar), so there is no universal default to
+guess here the way `xdg-open` covers `file_manager`; this action simply
+doesn't appear when the key is empty. Right-click bare desktop: "New
+Folder" (creates `~/Desktop/New Folder`, de-duplicated as `New Folder
+(2)`, `(3)`, ...) and "Refresh" (re-scans `~/Desktop`).
+
+Not implemented in this first pass, deliberately: moving a file to trash,
+emptying the trash, filesystem watching (a file added to `~/Desktop` by
+another program needs "Refresh" or a restart to appear), multi-select,
+and per-mimetype icon art. The first two are destructive/hard-to-reverse
+actions with no confirmation-dialog primitive to gate them on yet.
+
 ### Monitor Settings (`monitor.*`)
 ```lua
 srd.set("monitor.primary_layout", "dynamic")           -- Default: "dynamic"