feat(clients): render-scale setting on every client — shared punktfunk_core::render_scale

Client-side supersampling/downscaling: the client asks the host to render
and encode at chosen-resolution × scale (the host does no scaling) and the
presenter rescales the decoded frame to the display. >1 supersamples for
sharpness; <1 lightens the host GPU and the link. Default 1.0 = Native, the
prior behavior.

The geometry lives once in punktfunk_core::render_scale (multiply, preserve
aspect ratio, floor to even, clamp to the codec's per-axis ceiling — 4096
for H.264, 8192 otherwise), the Rust twin of the Apple client's
RenderScale.swift, consumed by the native session client, the presenter's
match-window path, the Windows/Linux settings UIs, Decky, and Android
(settings + host connect + unit test).

Implemented and platform-verified by the Apple-client-features session
(Linux+Android+Apple green there); the punktfunk-core wiring
(pub mod render_scale) is restored here after being lost in a working-tree
reconciliation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/decky/src/settings.tsx

@@ -25,6 +25,10 @@ const RESOLUTIONS: [number, number, string][] = [
   [2560, 1440, "2560 × 1440"],
 ];
 const REFRESH = [0, 30, 60, 90, 120];
+// Render-resolution multipliers (mirrors punktfunk_core::render_scale::PRESETS). 1.0 = native.
+const RENDER_SCALES = [0.5, 0.67, 0.75, 1.0, 1.25, 1.5, 2.0, 3.0, 4.0];
+const renderScaleLabel = (x: number): string =>
+  x === 1 ? "Native (1×)" : x > 1 ? `${x}× · supersample` : `${x}×`;
 const GAMEPADS = ["auto", "xbox360", "xboxone", "dualsense", "dualshock4", "steamdeck"];
 const GAMEPAD_LABELS: Record<string, string> = {
   auto: "Automatic",
@@ -106,6 +110,24 @@ export const SettingsSection: FC = () => {
           </div>
         </RowActions>
       </Field>
+      <Field
+        label="Render scale"
+        description="Supersample for sharpness (> 1×, more bandwidth) or render below native (< 1×) — the Deck resamples to its screen"
+        childrenContainerWidth="max"
+      >
+        <RowActions>
+          <div style={selectShell}>
+            <Dropdown
+              rgOptions={RENDER_SCALES.map((x) => ({ data: x, label: renderScaleLabel(x) }))}
+              // Snap the stored value to the nearest preset so the dropdown always shows a match.
+              selectedOption={RENDER_SCALES.reduce((best, x) =>
+                Math.abs(x - (s.render_scale ?? 1)) < Math.abs(best - (s.render_scale ?? 1)) ? x : best,
+              )}
+              onChange={(o) => patch({ render_scale: o.data as number })}
+            />
+          </div>
+        </RowActions>
+      </Field>
       <SliderField
         label="Bitrate"
         description="Mbit/s · 0 = host default"