Label the tiles

"title · app" centred under each thumbnail, in the same font the rofi
theme used (Berkeley Mono at pango's "small"), with the selected tile's
label inverted onto the yellow the way rofi's element background did.
Long titles are ellipsised to the cell width.

Text comes from cosmic-text, which brings real shaping. The catch is
cost: building a font system and rasterising the first glyphs takes
~55ms, nearly doubling a 65ms startup. But the capture phase is ~55ms of
sitting blocked while the compositor copies pixels, so labels are shaped
on a worker thread started before the captures and joined after them.
The measured labels phase is now 0.0ms — it costs nothing in wall clock.

Two thirds of that font cost was FontSystem::new() scanning all 1793
system faces, which even new_with_fonts() does. So the database is built
by hand: the user's own font directories first, since they are small,
and the full system scan only when the family isn't found there. An
unknown family still resolves, because that fallback is exactly the
system scan (verified: --font "No Such Font" renders in Noto).

--hide-labels restores the icon-only grid, and --font/--font-size make
the family and size settable. Adds a test that a label actually puts
pixels on the surface, one for ellipsising, and one pinning the label row
into the element geometry.

README.md

@@ -14,27 +14,35 @@ 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
+capture        52.5ms     8 windows, all frames in flight at once
+labels          0.0ms     shaped on a worker thread while the captures ran
+mapped          4.9ms     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.
+thumbnails are — which also makes it a free window to do other work in. Loading
+a font and rasterising its first glyphs costs ~20ms, so labels are shaped on a
+worker thread started before the captures and joined after them, and cost
+nothing in wall clock.
 
 ## Status
 
-Working, and usable as a switcher today: a static grid with keyboard navigation.
-Labels, filtering and live previews are next — see the roadmap.
+Working, and usable as a switcher today: a labelled static grid with keyboard
+navigation. Filtering and live previews are next — see the roadmap.
 
 ## Usage
 
 ```
-wlgrid [--print] [--verbose] [--timeout SECS]
+wlgrid [--print] [--verbose] [--hide-labels] [--font FAMILY] [--font-size PX]
+       [--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
+- `--hide-labels` draws an icon-only grid
+- `--font FAMILY` label font family (default `Berkeley Mono`)
+- `--font-size PX` label size in logical px
 - `--timeout SECS` exits after a deadline (an escape hatch: the overlay takes an
   exclusive keyboard grab)
 
@@ -60,8 +68,14 @@ cannot drive it. That goes away with xkb support, which filtering needs anyway.
 
 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`.
+columns capped at 4, 16:9 tiles, `title · app` centred underneath) and live in
+`src/theme.rs`. They will move to a config file so they can't drift from the
+`.rasi`.
+
+The font is looked up by family name. Your own font directories are scanned
+first because they are small; the full system scan (~37ms) happens only if the
+family isn't found there, and an unknown family then falls back to whatever
+cosmic-text picks rather than failing. Long titles are ellipsised to the cell.
 
 ## Requirements
 
@@ -79,7 +93,6 @@ 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`
@@ -89,5 +102,5 @@ unaffected. It matters more once previews are live.
 
 ```
 cargo build --release
-cargo test          # grid geometry
+cargo test          # grid geometry, ellipsising, glyph output
 ```