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/main.py

@@ -896,8 +896,8 @@ class Plugin:
         except (OSError, json.JSONDecodeError):
             # The client's own defaults (native display, host-default bitrate, auto pad).
             return {
-                "width": 0, "height": 0, "refresh_hz": 0, "bitrate_kbps": 0,
-                "codec": "auto", "gamepad": "auto", "compositor": "auto",
+                "width": 0, "height": 0, "refresh_hz": 0, "render_scale": 1.0,
+                "bitrate_kbps": 0, "codec": "auto", "gamepad": "auto", "compositor": "auto",
                 "inhibit_shortcuts": True, "mic_enabled": False,
             }