JR-004, JR-005, JR-006: scene-scoped read path

Presence was decided by a LINQ predicate inline in the controller, so the
semantics SR-002 sets were nowhere stated in code -- the read path complied by
accident rather than by requirement. PresenceLookup is now the unit that
decides, tagged, with the reasoning next to it.

JR-004: windows are served exactly as given. UT-021 pins that [0,10] and
[10,20] are not merged despite looking mergeable -- two windows mean a genuine
departure and return, and collapsing them answers a different question from the
one the truth file asked. UT-022 pins a byte-identical round trip.

JR-005: bounds inclusive at both ends, zero-length windows are real sightings
rather than degenerate ones to discard, overlaps resolve.

The wording was the larger half of JR-005. The overlay rendered a bare list: it
asserted nothing, but told the viewer nothing either, and the default reading of
a paused frame is "these people are on screen" -- exactly what SR-002 forbids.
It now carries an "In this scene" heading. ActorAtTime became ActorInScene, and
README no longer contains "on screen" anywhere; it stated the forbidden reading
outright in seven places, including the opening sentence.

JR-006: measured rather than assumed. UT-023 builds 50 actors x 1000 windows and
asserts the response is bounded by actor count, never window count. The lookup
is a full scan on purpose -- an early exit on `start > t` would exploit the
sortedness the format requires, but would silently under-report the moment one
producer emitted windows out of order. UT-020 pins that unsorted input still
resolves; WindowsAreSorted is a diagnostic, not a correctness dependency.

Third mutation check: making the end bound exclusive fails UT-016 and UT-018 and
nothing else. One character turns an inclusive window into a half-open one,
dropping an actor at exactly the moment a scene ends.

TRACES: UT-016, UT-017, UT-018, UT-019, UT-020, UT-021, UT-022, UT-023
TRACES: JR-004, JR-005, JR-006 | SR-002

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

Jellyfin.Plugin.JRay/Services/PresenceLookup.cs

@@ -0,0 +1,119 @@
+using System.Collections.Generic;
+using Jellyfin.Plugin.JRay.Models;
+
+namespace Jellyfin.Plugin.JRay.Services;
+
+/// <summary>
+/// Answers "which actors are in the scene at time <c>t</c>" from a truth file.
+/// </summary>
+/// <remarks>
+/// This is the unit that decides presence, so the scene-scoped semantics live
+/// here rather than being spread through the controller.
+///
+/// <para>
+/// <b>A window is a claim about scene membership, not a recognition event.</b>
+/// An actor who turns away, is occluded, or is off-camera while the shot cuts to
+/// whoever they are speaking to is still present. Two windows mean a genuine
+/// departure and return, not a break in detection — so this code reads windows
+/// exactly as given and never merges, splits, trims, or reorders them.
+/// </para>
+/// <para>
+/// Bounds are inclusive at both ends, matching the format's definition. That
+/// makes adjacent windows such as <c>[0,10]</c> and <c>[10,20]</c> both contain
+/// <c>t = 10</c>; reporting the actor present once is correct, and is not a
+/// reason to merge the windows.
+/// </para>
+/// </remarks>
+// TRACES: JR-004, JR-005, JR-006 | SR-002
+public static class PresenceLookup
+{
+    /// <summary>
+    /// Determines whether an actor is present in the scene at <paramref name="t"/>.
+    /// </summary>
+    /// <param name="actor">The actor entry from a truth file.</param>
+    /// <param name="t">The timestamp, in seconds.</param>
+    /// <returns><c>true</c> when any window contains <paramref name="t"/>.</returns>
+    public static bool IsPresentAt(TruthActor actor, double t)
+    {
+        if (actor is null)
+        {
+            return false;
+        }
+
+        // A full scan, deliberately: windows may be numerous, but correctness
+        // must not depend on the producer having honoured the sortedness
+        // guarantee. An early exit on `start > t` would be faster and would
+        // silently under-report the moment one file arrived out of order —
+        // trading a correctness risk for a saving that does not matter at this
+        // scale (see JR-006).
+        foreach (var window in actor.Scenes)
+        {
+            if (window.Length == 2 && window[0] <= t && t <= window[1])
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /// <summary>
+    /// Lists the actors present in the scene at <paramref name="t"/>, in the
+    /// order the truth file lists them.
+    /// </summary>
+    /// <param name="truth">The truth file.</param>
+    /// <param name="t">The timestamp, in seconds.</param>
+    /// <returns>The actors whose windows contain <paramref name="t"/>.</returns>
+    public static IEnumerable<TruthActor> ActorsPresentAt(TruthFile truth, double t)
+    {
+        if (truth is null)
+        {
+            yield break;
+        }
+
+        foreach (var actor in truth.Actors)
+        {
+            if (IsPresentAt(actor, t))
+            {
+                yield return actor;
+            }
+        }
+    }
+
+    /// <summary>
+    /// Determines whether an actor's windows are sorted by start time, as the
+    /// truth-file format requires of producers.
+    /// </summary>
+    /// <remarks>
+    /// Presence lookup does not depend on this — it is a diagnostic. A file that
+    /// fails it is still read correctly, but it signals a producer bug worth
+    /// surfacing rather than absorbing silently.
+    /// </remarks>
+    /// <param name="actor">The actor entry from a truth file.</param>
+    /// <returns><c>true</c> when every window starts at or after its predecessor.</returns>
+    public static bool WindowsAreSorted(TruthActor actor)
+    {
+        if (actor is null)
+        {
+            return true;
+        }
+
+        double previousStart = double.NegativeInfinity;
+        foreach (var window in actor.Scenes)
+        {
+            if (window.Length != 2)
+            {
+                continue;
+            }
+
+            if (window[0] < previousStart)
+            {
+                return false;
+            }
+
+            previousStart = window[0];
+        }
+
+        return true;
+    }
+}