feat(host,web): experimental DDC/CI monitor power-off for Exclusive sessions

The sole-virtual-display stutter investigation's active experiment: when the
Exclusive isolate deactivates a physical monitor, the dark-but-connected head
keeps getting serviced (monitor standby auto-input-scan / DP link churn) at a
seconds-scale cadence — the leading suspect for the periodic double-jolt. A
panel commanded off over DDC/CI (the VESA monitor-control channel in the video
cable) believes it has an owner and, on cooperating firmware, stops probing.

- New `ddc_power_off` display-policy axis (default off): orthogonal to presets
  like game_session, stored in display-settings.json, surfaced in GET/PUT
  /display/settings + the enforced list, carried through the layout transform.
- windows/ddc.rs: VCP 0xD6 power-mode control via the dxva2 Physical Monitor
  API. Deliberately DPMS-off (0x04, DDC stays responsive, signal return wakes)
  and never power-button-off (0x05, bricks-until-button on many monitors).
  Probe-before-write; every failure is skip-and-log — monitors without DDC/CI,
  OSD-disabled, or behind docks/KVMs degrade to a logged no-op.
- Manager wiring: panels commanded off immediately BEFORE the Exclusive CCD
  isolate (an HMONITOR — and with it the DDC channel — only exists while the
  display is active); teardown wakes them right after the CCD restore, where
  returning signal alone already wakes most firmware.
- Web console: an Experimental-badged on/off control on the display card,
  applied immediately like the game-session axis and preserved across preset
  switches; EN/DE strings incl. the wake-failure escape hatch (press the
  monitor's power button once, turn the option off).

Diagnostic value on top of the fix: if this kills a reporter's stutter, the
churn is monitor-firmware-initiated; if only topology=primary/extend does, the
driver services dark heads regardless — the two remaining root-cause classes.

Verified: Linux 258 tests + clippy + fmt clean; Windows (RTX box) 220/220 +
clippy clean; web tsc + production build clean; openapi.json regenerated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay/policy.rs

@@ -224,6 +224,16 @@ pub struct DisplayPolicy {
     /// so existing `display-settings.json` files are untouched.
     #[serde(default)]
     pub game_session: GameSession,
+    /// EXPERIMENTAL (Windows): command physical monitors' panels off over DDC/CI (VCP 0xD6 →
+    /// DPMS off) right before an `Exclusive` isolate deactivates them, and back on at restore.
+    /// Targets the "connected-but-dark head" periodic-stutter class (monitor standby
+    /// auto-input-scan / DP link churn while the virtual display is the sole active display) at
+    /// the monitor-firmware level. Best-effort — monitors without DDC/CI (or with it disabled in
+    /// the OSD) are skipped. Orthogonal to `preset` (like `game_session`): preserved across
+    /// preset changes; `#[serde(default)]` = off so existing `display-settings.json` files are
+    /// untouched.
+    #[serde(default)]
+    pub ddc_power_off: bool,
 }
 
 fn one() -> u32 {
@@ -247,6 +257,7 @@ impl Default for DisplayPolicy {
             layout: Layout::default(),
             max_displays: 4,
             game_session: GameSession::default(),
+            ddc_power_off: false,
         }
     }
 }
@@ -306,6 +317,7 @@ impl EffectivePolicy {
         &self,
         positions: BTreeMap<String, Position>,
         game_session: GameSession,
+        ddc_power_off: bool,
     ) -> DisplayPolicy {
         DisplayPolicy {
             version: 1,
@@ -319,8 +331,9 @@ impl EffectivePolicy {
                 positions,
             },
             max_displays: self.max_displays,
-            // Preserve the orthogonal game-session axis (EffectivePolicy doesn't carry it).
+            // Preserve the orthogonal axes (EffectivePolicy doesn't carry them).
             game_session,
+            ddc_power_off,
         }
     }
 }
@@ -434,6 +447,13 @@ impl DisplayPolicyStore {
         self.get().game_session
     }
 
+    /// The experimental DDC/CI panel-off axis — orthogonal to the preset (like
+    /// [`Self::game_session`]), read directly off the stored policy (default off when
+    /// unconfigured).
+    pub fn ddc_power_off(&self) -> bool {
+        self.get().ddc_power_off
+    }
+
     /// Persist + adopt a new policy (sanitized first). The in-memory value changes only if the disk
     /// write succeeds, so a full disk can't leave memory and file disagreeing.
     pub fn set(&self, policy: DisplayPolicy) -> Result<()> {
@@ -749,9 +769,10 @@ mod tests {
         let mut positions = BTreeMap::new();
         positions.insert("1".to_string(), Position { x: 0, y: 0 });
         positions.insert("7".to_string(), Position { x: 2560, y: 0 });
-        let p = eff.with_manual_layout(positions, GameSession::Dedicated);
-        // The orthogonal game-session axis is preserved through the layout transform.
+        let p = eff.with_manual_layout(positions, GameSession::Dedicated, true);
+        // The orthogonal axes (game-session, DDC power-off) are preserved through the transform.
         assert_eq!(p.game_session, GameSession::Dedicated);
+        assert!(p.ddc_power_off);
         // Preset drops to Custom so the explicit fields (incl. the layout) rule…
         assert_eq!(p.preset, Preset::Custom);
         // …every other behavior axis is preserved verbatim…
@@ -776,6 +797,8 @@ mod tests {
         assert_eq!(p.keep_alive, KeepAlive::default());
         assert_eq!(p.topology, Topology::Auto);
         assert_eq!(p.version, 1);
+        // A file written before the experimental DDC axis existed defaults it OFF.
+        assert!(!p.ddc_power_off);
     }
 
     #[test]