fix(quality): scale after videoconvert at exact even WxH

Live medium-quality stream errored with "negotiation problem" on the
host and rendered a squashed, garbled picture in the viewer. Two causes,
both from inserting videoscale before videoconvert with PAR+range caps:

- videoscale was scaling pipewiresrc's raw output directly. The portal
  source's format/memory (e.g. DMABuf) isn't something software videoscale
  negotiates — the original pipeline always fed pipewiresrc through
  videoconvert first. Move videoscale *after* videoconvert so it operates
  on system-memory NV12/I420.
- `pixel-aspect-ratio=1/1` + a width range over-constrained negotiation
  and risked a non-square-PAR / distorted result. Instead compute an exact
  even WxH from the known source dimensions (Wayland: portal size; X11:
  root/window geometry), preserving aspect, and pin it fully in the caps.
  This is also downscale-only now — a source already at/below the target
  height is left native instead of upscaled. Unknown dims (rare X11
  geometry failure) fall back to the height-only + square-pixel + even
  width-range negotiation.

source_dims threaded through pipeline::spawn from both backends. Smoke
test updated to mirror the new ordering (1920x1080 -> 852x480, videoscale
after videoconvert) and still asserts an even sub-source width.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/host/wayland.rs

@@ -70,7 +70,7 @@ pub async fn start(opts: &HostOpts, quality: &EffectiveQuality) -> Result<Captur
         "do-timestamp=true".to_string(),
     ];
 
-    pipeline::spawn(opts, quality, source_args, move || {
+    pipeline::spawn(opts, quality, Some((w as u32, h as u32)), source_args, move || {
         // Parent no longer needs the pipewire fd — gst inherited its own copy.
         let _ = close(raw_fd);
     })