iris: a degenerate fit has no solution, and the desktop follows a display's density

Two of docs/REVIEW-2026-09-07.md's risks.

**R7.** `poly_fit_least_squares` clamped a near-zero basis-vector norm
(`1.0 / dot(..).sqrt().max(1e-6)`) where Compose's `polyFitLeastSquares`
bails: below `0.000001f` the vectors are linearly dependent and there is
no solution. Clamping reached the solve with a `q` row of zeros and a
zero on `r`'s diagonal, produced `[NaN, NaN, NaN]`, and was rescued only
by the caller's `is_finite` check -- working, but by accident, and not
what the source it is transcribed from does. It returns `Option` now and
`velocity()` answers 0 on `None`.
`a_fit_through_linearly_dependent_points_has_no_solution` reports
`Some([NaN, NaN, NaN])` with the clamp back in place. Three samples at
one instant is exactly what the input clock produced before 2ec0fee, so
this is the second half of the same fault.

**R5.** `WindowEvent::ScaleFactorChanged` was unhandled, so dragging the
window to a display with a different scale left every `Len::dp` and every
rasterised glyph at the density the window opened on. It now re-reads
`content_scale` -- through that function rather than off the event, so
`IRIS_SCALE` still pins `--phone`'s density instead of following the
monitor -- and sets both copies. `UiRenderState::set_density` marks the
tree for a full redraw when the value actually changes, because
`Text::shape` keys its cache on `(attrs, width, density)` and nothing
else would ask for those glyphs again. Invisible on this machine (every
display here is 1.0), which is why the review asked for it in writing.

Verified: `cargo test --lib -p iris` (104), `cargo test -p
transcript-fixture` (12), `cargo ndk check -p iris`, fmt and clippy
clean, and layer 2 (`run-headless.sh phone --phone --replay
flick-120hz.touch --shot`) still draws and still clips at the composer.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

iris/core/src/ui/render_state.rs

@@ -167,7 +167,16 @@ impl UiRenderState {
     /// different triggers (a surface resize on every rotation or keyboard
     /// open; a density change only if the app follows the display to a
     /// different screen, which Android surfaces separately).
+    ///
+    /// Marks the tree for a full redraw when the value actually changes:
+    /// every `Len::dp` already resolved and every glyph already shaped
+    /// (`Text::shape` keys its cache on `(attrs, width, density)`) belongs
+    /// to the old one, and nothing else would ask for them again
+    /// (docs/REVIEW-2026-09-07.md's R5).
     pub fn set_density(&mut self, density: f32) {
+        if density != self.density {
+            self.resized = true;
+        }
         self.density = density;
     }