fix: m0 portal capture — activate the capturer so frames are delivered

The M2 teardown work added an `active` gate to the PipeWire capture callback
(idle by default so reconnects stay cheap, with the stream path calling
set_active(true) on PLAY). The `m0` subcommand was never updated, so its portal
capturer stayed inactive and the callback dropped every frame — `m0 --source
portal` failed with "no PipeWire frame within 10s" on every compositor. Call
set_active(true) before the capture loop.

Validated on headless KWin (Plasma 6.4) via the RemoteDesktop-anchored ScreenCast
session: real desktop frames flow (shm BGRx 1920x1080) and encode to valid H.265.

(Also folds in a rustfmt reflow of the input-test log line.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/lumen-host/src/m0.rs

@@ -59,6 +59,10 @@ pub fn run(opts: Options) -> Result<()> {
         }
     };
 
+    // Activate the capturer so the portal/PipeWire process callback actually delivers frames
+    // (it gates the per-frame de-pad on `active`; idle by default so reconnects are cheap).
+    capturer.set_active(true);
+
     // The first frame establishes the authoritative dimensions (the portal's negotiated
     // size, or the synthetic size) used to configure the encoder.
     let first = capturer.next_frame().context("capture first frame")?;