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>

web/src/sections/Displays/DisplayCard.tsx

@@ -165,8 +165,9 @@ const DisplayForm: FC<{
 				identity: effective.identity,
 				layout: effective.layout,
 				max_displays: effective.max_displays,
-				// Game-session is orthogonal to the preset — carry it through the Custom switch.
+				// Game-session + DDC are orthogonal to the preset — carry them through the Custom switch.
 				game_session: draft.game_session ?? "auto",
+				ddc_power_off: draft.ddc_power_off ?? false,
 			});
 		} else {
 			apply({ ...draft, preset: id as Preset });
@@ -181,6 +182,8 @@ const DisplayForm: FC<{
 			preset: "custom",
 			...p.fields,
 			game_session: p.game_session ?? "auto",
+			// The experimental DDC axis isn't part of a preset — keep the current setting.
+			ddc_power_off: draft.ddc_power_off ?? false,
 		});
 
 	// A custom card is "current" when the in-force policy is a Custom one whose fields + game-session
@@ -464,6 +467,37 @@ const DisplayForm: FC<{
 				/>
 			</div>
 
+			{/* EXPERIMENTAL: DDC/CI panel power-off — orthogonal like game-session (survives preset
+			    switches, applies immediately). Windows-only in effect, acted on at the Exclusive isolate. */}
+			<div className="border-t pt-4">
+				<div className="space-y-3">
+					<div className="flex items-center gap-2">
+						<Label className="block">{m.display_ddc()}</Label>
+						<Badge variant="outline" className="text-amber-600 dark:text-amber-500">
+							{m.display_experimental()}
+						</Badge>
+					</div>
+					<div className="flex flex-wrap gap-2">
+						{([false, true] as const).map((on) => (
+							<Button
+								key={String(on)}
+								size="sm"
+								variant={(draft.ddc_power_off ?? false) === on ? "default" : "outline"}
+								disabled={busy}
+								onClick={() => {
+									const next = { ...draft, ddc_power_off: on };
+									setDraft(next);
+									apply(next);
+								}}
+							>
+								{on ? m.display_ddc_enabled() : m.display_ddc_disabled()}
+							</Button>
+						))}
+					</div>
+					<p className="max-w-prose text-xs text-muted-foreground">{m.display_ddc_help()}</p>
+				</div>
+			</div>
+
 			{/* What's in force right now */}
 			<div className="flex flex-wrap items-center gap-2 border-t pt-3">
 				<span className="text-sm text-muted-foreground">{m.display_effective()}:</span>
@@ -476,6 +510,9 @@ const DisplayForm: FC<{
 				{(draft.game_session ?? "auto") === "dedicated" && (
 					<Badge variant="secondary">{m.display_game_session_dedicated()}</Badge>
 				)}
+				{(draft.ddc_power_off ?? false) && (
+					<Badge variant="outline">{m.display_ddc_badge()}</Badge>
+				)}
 			</div>
 
 			<p className="max-w-prose text-xs text-muted-foreground">{m.display_pending_note()}</p>