feat(host/windows): run serve/m3-host on Windows (config paths + compositor)

The punktfunk/1 control plane already compiled on Windows; these wire the last gaps so the host actually runs: config_dir falls back to %APPDATA% (HOME\.config when set), paired_path uses it, hostname from COMPUTERNAME, and resolve_compositor short-circuits the Linux session-detection on Windows (SudoVDA is the single backend; vdisplay::open ignores the compositor arg). Validated live on the VM: m3-host creates its identity, binds the QUIC endpoint (fingerprint logged), advertises mDNS (_punktfunk._udp, host from COMPUTERNAME), and accepts sessions. GPU-less validations green: m0 synthetic->openh264->core FEC loopback (120/120, 0 mismatches) and the m3 c_abi_connection_roundtrip control-plane test. Full session capture (SudoVDA->DXGI) + NVENC remain GPU-gated.

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

crates/punktfunk-host/src/m3.rs

@@ -1519,6 +1519,15 @@ fn pick_compositor(
 /// async reactor (`spawn_blocking`).
 fn resolve_compositor(pref: CompositorPref) -> Result<crate::vdisplay::Compositor> {
     use crate::vdisplay::Compositor;
+    // Windows has a single virtual-display backend (SudoVDA); vdisplay::open ignores the compositor
+    // arg there, so short-circuit the Linux session-detection state machine with a placeholder.
+    #[cfg(target_os = "windows")]
+    {
+        let _ = pref;
+        return Ok(Compositor::Kwin);
+    }
+    #[cfg(not(target_os = "windows"))]
+    {
     // Explicit operator override (legacy / CI / forcing a backend for a test) wins and is assumed
     // to come with a hand-set env — don't retarget the process env in that case.
     let overridden = std::env::var_os("PUNKTFUNK_COMPOSITOR").is_some();
@@ -1565,6 +1574,7 @@ fn resolve_compositor(pref: CompositorPref) -> Result<crate::vdisplay::Composito
         ),
     }
     Ok(chosen)
+    }
 }
 
 /// Bounds a speed-test [`ProbeRequest`] before bursting: a 3 Gbps / 5 s ceiling keeps a probe from