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/backend.ts

@@ -99,6 +99,7 @@ export interface StreamSettings {
   width: number; // 0 = native
   height: number; // 0 = native
   refresh_hz: number; // 0 = native
+  render_scale?: number; // render-resolution multiplier; 1.0 = native (absent in pre-scale files)
   bitrate_kbps: number; // 0 = host default
   codec?: string; // "auto" | "hevc" | "h264" | "av1" — soft preference (absent in pre-codec files)
   gamepad: string; // "auto" | "xbox360" | "xboxone" | "dualsense" | "dualshock4" | "steamdeck"