feat: the streamed screen is a persisted setting, pickable from the console

P4 of design/per-monitor-portal-capture.md. The pin was an env var read
once at startup, which a console picker can never write — so it becomes a
field of the display policy (orthogonal to presets, like game_session), and
`vdisplay::capture_monitor()` resolves env-over-policy: an appliance that
pinned in its unit's host.env stays pinned there, and a console click
cannot re-aim a machine whose operator already declared the answer.

Read per `open` rather than cached, so a picker change takes effect on the
next session instead of the next host restart. The host re-resolves the
input anchor on every policy write — including clearing it when the pin is
cleared, or a later virtual-display session inherits an anchor aimed at a
monitor it is not showing.

`with_manual_layout` carries the pin through like the other orthogonal
axes: without that, saving a display ARRANGEMENT would silently swap the
streamed screen back to virtual.

Console: a "Streamed screen" card listing the host's real monitors beside
"Virtual screen (default)", saving on selection. A disabled head is listed
but not selectable (so "why isn't it here?" has an answer), and an
env-pinned host renders read-only with the reason rather than offering
controls that silently lose to the environment.

Verified on GNOME/Mutter: pinning via the policy file alone (no env) routes
sessions to the mirror and anchors input; setting the env to a different
connector overrides it, exactly as documented.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/devtest.rs

@@ -502,7 +502,7 @@ pub fn mirror_test(args: &[String]) -> Result<()> {
     let explicit = arg("--monitor");
     let want = explicit
         .clone()
-        .or_else(|| pf_host_config::config().capture_monitor.clone())
+        .or_else(crate::vdisplay::capture_monitor)
         .context(
             "no monitor named — pass --monitor <CONNECTOR> or set PUNKTFUNK_CAPTURE_MONITOR",
         )?;