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/messages/de.json

@@ -102,6 +102,12 @@
 	"display_game_session_help": "Wie eine Sitzung bedient wird, die ein Spiel aus der Bibliothek startet. „Dediziert“ gibt dem Start immer ein eigenes headless-gamescope in genau deiner Auflösung — das Spiel startet direkt, ohne Steam Big Picture, ohne Game-Mode. „Auto“ nutzt die aktuelle Sitzung der Box. gamescope muss installiert sein; sonst fällt Dediziert auf Auto zurück.",
 	"display_game_session_auto": "Auto",
 	"display_game_session_dedicated": "Dediziert",
+	"display_experimental": "Experimentell",
+	"display_ddc": "Monitore beim Streamen ausschalten (DDC/CI)",
+	"display_ddc_help": "Nur Windows, wirkt bei Topologie Exklusiv. Bevor die physischen Monitore deaktiviert werden, weist der Host sie zusätzlich über den DDC/CI-Steuerkanal an, ihr Panel auszuschalten — und weckt sie am Ende des Streams wieder. Das kann ein periodisches Ruckeln beheben, das manche Setups zeigen, wenn die gestreamte Anzeige die einzige aktive ist (der dunkle Monitor sucht sonst weiter seine Eingänge ab; ein schlafen gelegtes Panel nicht). Monitore ohne DDC/CI werden übersprungen — falls ein Monitor danach nicht aufwacht, einmal seinen Power-Knopf drücken und die Option ausschalten.",
+	"display_ddc_disabled": "Aus",
+	"display_ddc_enabled": "Ein",
+	"display_ddc_badge": "Monitore aus via DDC/CI",
 	"display_identity_shared": "Geteilt",
 	"display_identity_per_client": "Pro Client",
 	"display_identity_per_client_mode": "Pro Client + Auflösung",

web/messages/en.json

@@ -102,6 +102,12 @@
 	"display_game_session_help": "How a session that launches a game from the library is served. “Dedicated” always gives the launch its own headless gamescope at your exact resolution — the game boots straight in, no Steam Big Picture, no game mode. “Auto” uses whatever session the box is in. gamescope must be installed; otherwise Dedicated falls back to Auto.",
 	"display_game_session_auto": "Auto",
 	"display_game_session_dedicated": "Dedicated",
+	"display_experimental": "Experimental",
+	"display_ddc": "Turn monitors off while streaming (DDC/CI)",
+	"display_ddc_help": "Windows only, takes effect with Exclusive topology. Before disabling your physical monitors, the host also tells them to power their panel off over the DDC/CI monitor-control channel, and wakes them again when the stream ends. This can eliminate a periodic stutter some setups see when the streamed display is the only active one (the dark monitor keeps probing its inputs; a panel that was told to sleep doesn't). Monitors without DDC/CI support are skipped — if your monitor doesn't wake up afterwards, press its power button once and turn this off.",
+	"display_ddc_disabled": "Off",
+	"display_ddc_enabled": "On",
+	"display_ddc_badge": "Monitors off via DDC/CI",
 	"display_identity_shared": "Shared",
 	"display_identity_per_client": "Per client",
 	"display_identity_per_client_mode": "Per client + resolution",