fix(servo): enable CSS Grid so grid layouts stop collapsing

`Preferences::default()` is Servo's conservative library default and
ships `layout_grid_enabled: false`. `Servo::new` forwards prefs to Stylo
(`prefs::set` -> `stylo_static_prefs::set_pref!("layout.grid.enabled")`),
so with the gate off Stylo blockifies `display: grid`: every grid
container collapses to `display: block` and grid-based page layouts
stack into a single column — the "broken" rendering reported on modern
sites.

`ely_servo_preferences()` only flipped `dom_intersection_observer_enabled`
and inherited the grid default, so ELY rendered grid pages collapsed
while Servo's own servoshell (which enables the pref) renders them
correctly. The layout path is implemented — servo-layout drives
`DisplayInside::Grid` through Taffy — so enabling the pref is the real
fix, not a workaround.

Verified with a deterministic `display: grid; grid-template-columns:
1fr 1fr 1fr` page: 9 stacked full-width bars before, a 3x3 grid after.
Wikipedia/HN/GitHub/google.com re-checked unchanged; full workspace
test suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/ely_servo_host/src/runtime.rs

@@ -241,7 +241,18 @@ fn install_rustls_provider() {
 }
 
 fn ely_servo_preferences() -> Preferences {
-    Preferences { dom_intersection_observer_enabled: true, ..Preferences::default() }
+    // `Preferences::default()` is Servo's conservative *library* default and ships
+    // CSS Grid off. `Servo::new` forwards prefs to Stylo via `prefs::set`, and with
+    // `layout.grid.enabled` off Stylo blockifies `display: grid`, collapsing every
+    // grid container to `display: block` — modern grid layouts stack into one column
+    // (the "broken" rendering on grid-based sites). The layout path is implemented
+    // (servo-layout drives `DisplayInside::Grid` through Taffy), so an embedder
+    // building a browser must enable it, exactly as Servo's own servoshell does.
+    Preferences {
+        dom_intersection_observer_enabled: true,
+        layout_grid_enabled: true,
+        ..Preferences::default()
+    }
 }
 
 impl ServoHost for SoftwareServoHost {