fix(displays): verify DDC/CI writes and trust capability string for picture mode

DDC/CI writes are fire-and-forget at the protocol level: a successful send
does not mean the monitor honored the value. Many monitors (LG ultrawides
in particular) silently drop writes for VCP codes whose registers exist
but whose feature isn't really implemented in firmware.

- New _verify_after_set helper polls readback after every DDC/CI write and
  reports {success: false} when the monitor didn't apply the value. Wired
  into set_contrast, set_input_source, set_color_preset, set_picture_mode.
  Input source uses a longer settle window since switching can briefly
  disrupt the DDC/CI link.

- Picture mode (VCP 0xDC) now requires the capability string to declare
  supported codes under cmds[0xDC]. Without that declaration we treat the
  feature as unsupported even when reads succeed - the LG case where reads
  return a stuck value and every write is silently ignored.

media_server/services/display_service.py

@@ -267,20 +267,27 @@ def _probe_static_open(mon, mc, monitor_id: int) -> dict:
     except Exception as e:
         logger.debug("Monitor %d: color_preset unsupported: %s", monitor_id, e)
 
-    # Picture / scene mode (VCP 0xDC) — not exposed by monitorcontrol's
-    # high-level API, so probe via raw VCP transport.
-    try:
-        mon.vcp.get_vcp_feature(PICTURE_MODE_VCP)
-        static["picture_mode_supported"] = True
-        cmds = caps.get("cmds") or {}
-        declared = cmds.get(PICTURE_MODE_VCP)
-        codes = sorted(declared) if declared else sorted(PICTURE_MODE_LABELS.keys())
-        static["available_picture_modes"] = [
-            {"code": c, "label": PICTURE_MODE_LABELS.get(c, f"Mode {c}")}
-            for c in codes
-        ]
-    except Exception as e:
-        logger.debug("Monitor %d: picture_mode unsupported: %s", monitor_id, e)
+    # Picture / scene mode (VCP 0xDC). Trickier than color preset because
+    # many monitors (LG ultrawides included) respond to READS but silently
+    # drop every WRITE - they implement the register but not the feature.
+    # The capability string is the most reliable signal: a monitor that
+    # really implements picture mode declares its supported codes under
+    # cmds[0xDC]. If 0xDC isn't declared, treat the feature as unsupported
+    # to avoid exposing a non-functional select.
+    cmds = caps.get("cmds") or {}
+    declared = cmds.get(PICTURE_MODE_VCP)
+    if declared:
+        try:
+            mon.vcp.get_vcp_feature(PICTURE_MODE_VCP)
+            static["picture_mode_supported"] = True
+            static["available_picture_modes"] = [
+                {"code": c, "label": PICTURE_MODE_LABELS.get(c, f"Mode {c}")}
+                for c in sorted(declared)
+            ]
+        except Exception as e:
+            logger.debug("Monitor %d: picture_mode declared but unreadable: %s", monitor_id, e)
+    else:
+        logger.debug("Monitor %d: picture_mode (VCP 0xDC) not declared in capability string", monitor_id)
 
     return static
 
@@ -498,6 +505,31 @@ def set_power(monitor_id: int, on: bool) -> bool:
         return False
 
 
+def _verify_after_set(getter, expected, *, retries: int = 3, delay: float = 0.1) -> bool:
+    """Poll a DDC/CI getter to confirm the monitor actually applied a write.
+
+    DDC/CI writes are fire-and-forget at the protocol level: a successful
+    send does not mean the monitor honored the value. Many monitors silently
+    drop writes for codes their firmware doesn't really implement (LG's
+    ColorPreset / Picture Mode are common offenders). Without this check the
+    API would report `success: true` while the monitor sat unchanged.
+
+    Compares both raw and `.value` forms so enum/int mismatches don't flag a
+    spurious failure.
+    """
+    expected_int = getattr(expected, "value", expected)
+    for _ in range(retries):
+        time.sleep(delay)
+        try:
+            actual = getter()
+        except Exception:
+            continue
+        actual_int = getattr(actual, "value", actual)
+        if actual == expected or actual_int == expected_int:
+            return True
+    return False
+
+
 def set_contrast(monitor_id: int, value: int) -> bool:
     """Set contrast for a specific monitor (0-100) via DDC/CI."""
     mc = _load_monitorcontrol()
@@ -511,6 +543,9 @@ def set_contrast(monitor_id: int, value: int) -> bool:
             return False
         with ddc_monitors[monitor_id] as monitor:
             monitor.set_contrast(value)
+            if not _verify_after_set(monitor.get_contrast, value):
+                logger.warning("Monitor %d: contrast %d not applied", monitor_id, value)
+                return False
         _invalidate_cache()
         return True
     except Exception as e:
@@ -536,6 +571,11 @@ def set_input_source(monitor_id: int, source: str) -> bool:
             return False
         with ddc_monitors[monitor_id] as monitor:
             monitor.set_input_source(target)
+            # Source switches can briefly disrupt the DDC/CI link; allow a
+            # longer settle window before declaring failure.
+            if not _verify_after_set(monitor.get_input_source, target, retries=5, delay=0.2):
+                logger.warning("Monitor %d: input source %s not applied", monitor_id, source)
+                return False
         _invalidate_cache()
         return True
     except Exception as e:
@@ -561,6 +601,12 @@ def set_color_preset(monitor_id: int, preset: str) -> bool:
             return False
         with ddc_monitors[monitor_id] as monitor:
             monitor.set_color_preset(target)
+            if not _verify_after_set(monitor.get_color_preset, target):
+                logger.warning(
+                    "Monitor %d: color preset %s not applied (monitor silently rejected)",
+                    monitor_id, preset,
+                )
+                return False
         _invalidate_cache()
         return True
     except Exception as e:
@@ -584,6 +630,16 @@ def set_picture_mode(monitor_id: int, code: int) -> bool:
             return False
         with ddc_monitors[monitor_id] as monitor:
             monitor.vcp.set_vcp_feature(PICTURE_MODE_VCP, code)
+            # Raw VCP read returns (current, maximum) — only compare current.
+            def _read_picture_mode():
+                current, _ = monitor.vcp.get_vcp_feature(PICTURE_MODE_VCP)
+                return current
+            if not _verify_after_set(_read_picture_mode, code):
+                logger.warning(
+                    "Monitor %d: picture mode code %d not applied (monitor silently rejected)",
+                    monitor_id, code,
+                )
+                return False
         _invalidate_cache()
         return True
     except Exception as e: