feat(quality): log the actual encode resolution at capture spawn

Window size in the viewer is an unreliable proxy for the encoded
resolution (mpv clamps/scales to the screen), making it hard to tell
whether a preset's downscale actually took effect. Log the concrete
decision host-side when capture spawns:

- "downscaling video from=1920x1080 to=1280x720" when scaling,
- "encoding at native resolution" for Source,
- "source already at/below preset height" when no upscale is needed,
- the unknown-dims fallback case too.

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

src/host/pipeline.rs

@@ -202,19 +202,43 @@ fn build_args(
     // X11 geometry-read failure) we fall back to height-only + square pixels +
     // an even-stepped width range and let videoscale negotiate.
     let scale_caps: Option<String> = match quality.max_height {
-        None => None,
+        None => {
+            tracing::info!(preset = %quality.label, "encoding at native resolution (no downscale)");
+            None
+        }
         Some(max_h) => {
             let h = (max_h & !1).max(2);
             match source_dims {
                 Some((sw, sh)) if sh > h => {
                     let w = ((sw as u64 * h as u64 + sh as u64 / 2) / sh as u64) as u32;
                     let w = (w & !1).max(2);
+                    tracing::info!(
+                        preset = %quality.label,
+                        from = %format!("{sw}x{sh}"),
+                        to = %format!("{w}x{h}"),
+                        "downscaling video"
+                    );
                     Some(format!("{raw_format},width={w},height={h}"))
                 }
-                Some(_) => None, // source already <= target height
-                None => Some(format!(
-                    "{raw_format},height={h},pixel-aspect-ratio=1/1,width=[2,8192,2]"
-                )),
+                Some((sw, sh)) => {
+                    tracing::info!(
+                        preset = %quality.label,
+                        source = %format!("{sw}x{sh}"),
+                        max_height = h,
+                        "source already at/below preset height — encoding native (no upscale)"
+                    );
+                    None
+                }
+                None => {
+                    tracing::info!(
+                        preset = %quality.label,
+                        max_height = h,
+                        "downscaling to max height (source size unknown — width follows negotiation)"
+                    );
+                    Some(format!(
+                        "{raw_format},height={h},pixel-aspect-ratio=1/1,width=[2,8192,2]"
+                    ))
+                }
             }
         }
     };