Add wlgrid: a window-thumbnail grid overlay for wlroots

A switcher to replace a wlthumbs + rofi pipeline, with the same look
(gruvbox, ceil(sqrt(n)) columns capped at 4, 16:9 tiles, a yellow
selection filling the element padding) but no thumbnails anywhere.

Each window is captured straight into a wl_shm buffer that is handed to
its own wl_subsurface, with wp_viewporter giving the compositor the
rectangle to scale it into. So there is no PNG encode, no scaler, no
full-resolution bitmap in this process, and the capture buffers are never
even mapped here — the compositor writes those pages and samples them
again for display. Opens in ~65ms for 8 windows (55ms of which is the
compositor reading pixels back out of the GPU) and holds ~9MB of RSS.

All capture sessions are opened before a single roundtrip and every frame
goes in flight together, the same batching wlthumbs uses, because the
readback is bandwidth-bound rather than latency-bound.

sway remains the source of truth: the window list, the con_ids and the
focusing all come from its IPC socket, joined to the Wayland side by
foreign_toplevel_identifier. Navigation reads raw evdev keycodes so it is
layout-independent, which does mean virtual-keyboard clients that invent
their own keymap can't drive it; that resolves when filtering brings xkb.

Labels, type-to-filter and live previews are next.

README.md

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+# wlgrid
+
+A window switcher for wlroots compositors: a thumbnail grid overlay that looks
+like a rofi theme, and focuses the window you pick.
+
+It replaces a `wlthumbs | rofi` pipeline. The difference is that no thumbnails
+exist: each window is captured straight into a `wl_shm` buffer that is handed to
+its own `wl_subsurface`, and `wp_viewporter` tells the compositor which rectangle
+to scale it into. There is no image encoding, no scaler, and no full-resolution
+bitmap in this process — which is also why it holds ~9 MB of RSS and appears in
+about 60 ms.
+
+```
+sway-tree       0.6ms     window list + con_ids over sway IPC
+toplevels       0.2ms     ext-foreign-toplevel-list handles
+constraints     1.5ms     every capture session's buffer size, in one roundtrip
+capture        55.0ms     8 windows, all frames in flight at once
+mapped          4.2ms     layer surface + subsurfaces on screen
+```
+
+The capture phase is the compositor reading full-resolution window pixels out of
+the GPU. It is bandwidth-bound (~1.1 GB/s here) and unaffected by how large the
+thumbnails are.
+
+## Status
+
+Working, and usable as a switcher today: a static grid with keyboard navigation.
+Labels, filtering and live previews are next — see the roadmap.
+
+## Usage
+
+```
+wlgrid [--print] [--verbose] [--timeout SECS]
+```
+
+- `--print` writes the selected sway `con_id` to stdout instead of focusing it
+- `--verbose` prints phase timings and how many windows were captured
+- `--timeout SECS` exits after a deadline (an escape hatch: the overlay takes an
+  exclusive keyboard grab)
+
+Bind it in sway:
+
+```
+bindsym $mod+Tab exec wlgrid
+```
+
+| key | |
+|---|---|
+| `→` `←` / `Tab` `Shift+Tab` | next / previous window |
+| `↑` `↓` | move a row |
+| `Home` `End` | first / last |
+| `Enter` | focus the selection |
+| `Escape` / `q` | cancel, leaving focus alone |
+
+Navigation reads raw evdev keycodes, so it is layout-independent — but it also
+means virtual-keyboard clients such as `wtype` (which invent their own keymap)
+cannot drive it. That goes away with xkb support, which filtering needs anyway.
+
+## Look
+
+Colours, font metrics and grid geometry come from the rofi theme this replaces
+(gruvbox dark, a yellow selection filling the element padding, `ceil(sqrt(n))`
+columns capped at 4, 16:9 tiles) and live in `src/theme.rs`. They will move to a
+config file so they can't drift from the `.rasi`.
+
+## Requirements
+
+A wlroots compositor advertising `ext-image-copy-capture-v1`,
+`ext-image-capture-source-v1` (with the foreign-toplevel source manager),
+`ext-foreign-toplevel-list-v1`, `wlr-layer-shell-unstable-v1` and
+`wp_viewporter` — sway 1.11+, and in principle Hyprland, labwc and jay, though
+only sway is tested. sway is also the source of truth for the window list and
+for focusing, over its IPC socket.
+
+Known upstream issue: holding per-toplevel capture sessions open makes windows
+blurry on **fractionally scaled** outputs
+([sway#9113](https://github.com/swaywm/sway/issues/9113)). Integer scales are
+unaffected. It matters more once previews are live.
+
+## Roadmap
+
+- **M2** labels: real text via cosmic-text, `--hide-labels` for an icon-only grid
+- **M3** type-to-filter with fzf-quality fuzzy matching (and xkb keyboard input)
+- **M4** live previews: keep the capture sessions open and re-capture on a rate
+  limit, `--live all|current|none`
+- **M5** dmabuf capture, so the pixels never leave the GPU at all
+
+## Building
+
+```
+cargo build --release
+cargo test          # grid geometry
+```