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.Tests/PresenceLookupTests.cs

@@ -0,0 +1,150 @@
+using System.Diagnostics;
+using System.Linq;
+using System.Text.Json;
+using Jellyfin.Plugin.JRay.Models;
+using Jellyfin.Plugin.JRay.Services;
+using Xunit;
+
+namespace Jellyfin.Plugin.JRay.Tests;
+
+/// <summary>
+/// JR-004 (windows are scene-membership claims, served verbatim), JR-005 (query
+/// semantics and inclusive bounds) and JR-006 (numerous windows).
+///
+/// These pin the semantics SR-002 sets. The failure they exist to prevent is a
+/// well-meaning "tidy-up" — merging adjacent windows, trimming a zero-length
+/// one, or collapsing overlaps — each of which silently answers a different
+/// question from the one the truth file asked.
+///
+/// TRACES: UT-016, UT-017, UT-018, UT-019, UT-020, UT-021, UT-022, UT-023 | JR-004, JR-005, JR-006
+/// </summary>
+public class PresenceLookupTests
+{
+    private static TruthActor Actor(params double[][] windows)
+    {
+        var actor = new TruthActor { Name = "Steve Buscemi", TmdbId = "884" };
+        foreach (var w in windows)
+        {
+            actor.Scenes.Add(w);
+        }
+
+        return actor;
+    }
+
+    // UT-016
+    [Theory]
+    [InlineData(12.0)]   // exactly the start
+    [InlineData(30.0)]   // inside
+    [InlineData(45.0)]   // exactly the end
+    public void IsPresentAt_WithinInclusiveBounds_IsPresent(double t)
+    {
+        // Both ends inclusive: a window is [start, end], not [start, end).
+        Assert.True(PresenceLookup.IsPresentAt(Actor([12.0, 45.0]), t));
+    }
+
+    // UT-017
+    [Theory]
+    [InlineData(11.999)]
+    [InlineData(45.001)]
+    public void IsPresentAt_OutsideBounds_IsAbsent(double t)
+    {
+        Assert.False(PresenceLookup.IsPresentAt(Actor([12.0, 45.0]), t));
+    }
+
+    // UT-018
+    [Fact]
+    public void IsPresentAt_ZeroLengthWindow_IsPresentAtThatInstant()
+    {
+        // A single sighting is a legitimate window. Discarding it as degenerate
+        // would drop the actor from a scene they are demonstrably in.
+        Assert.True(PresenceLookup.IsPresentAt(Actor([30.0, 30.0]), 30.0));
+    }
+
+    // UT-019
+    [Fact]
+    public void IsPresentAt_OverlappingWindows_IsPresentInsideTheEnclosingOne()
+    {
+        // [0,100] encloses [50,60]. A lookup that assumed non-overlapping,
+        // sorted windows and stopped at the first start > t would miss t = 80.
+        Assert.True(PresenceLookup.IsPresentAt(Actor([0.0, 100.0], [50.0, 60.0]), 80.0));
+    }
+
+    // UT-020
+    [Fact]
+    public void IsPresentAt_UnsortedWindows_StillFindsPresence()
+    {
+        // Sortedness is a producer guarantee, not something correctness may
+        // depend on. A file that violates it must still be read correctly.
+        var actor = Actor([100.0, 110.0], [10.0, 20.0]);
+
+        Assert.True(PresenceLookup.IsPresentAt(actor, 15.0));
+        Assert.False(PresenceLookup.WindowsAreSorted(actor));
+    }
+
+    // UT-021
+    [Fact]
+    public void ActorsPresentAt_AdjacentWindowsAreNeverMerged()
+    {
+        // [0,10] and [10,20] look mergeable. They must not be merged: two
+        // windows mean a genuine departure and return, and the plugin does not
+        // reinterpret that claim. The actor is reported once, from two windows.
+        var truth = new TruthFile();
+        truth.Actors.Add(Actor([0.0, 10.0], [10.0, 20.0]));
+
+        Assert.Single(PresenceLookup.ActorsPresentAt(truth, 10.0));
+        Assert.Equal(2, truth.Actors[0].Scenes.Count);
+    }
+
+    // UT-022
+    [Fact]
+    public void TruthFile_RoundTrips_WithWindowsByteIdentical()
+    {
+        // JR-004: served exactly as given. A round trip through the serializer
+        // is where a silent normalisation would show up.
+        const string Json = """
+        {"schema_version":1,"movie":"/m.mkv","sample_fps":1,"anneal_sec":2,
+         "actors":[{"name":"A","imdb_id":"","tmdb_id":"884","jellyfin_id":"",
+                    "scenes":[[0.0,10.0],[10.0,20.0],[30.0,30.0]]}]}
+        """;
+
+        var parsed = JsonSerializer.Deserialize<TruthFile>(Json, new JsonSerializerOptions(JsonSerializerDefaults.Web))!;
+        var windows = parsed.Actors[0].Scenes;
+
+        Assert.Equal(3, windows.Count);
+        Assert.Equal([0.0, 10.0], windows[0]);
+        Assert.Equal([10.0, 20.0], windows[1]);
+        Assert.Equal([30.0, 30.0], windows[2]);
+    }
+
+    // UT-023
+    [Fact]
+    public void ActorsPresentAt_WithManyWindows_StaysCheapAndBoundsTheResponse()
+    {
+        // SR-002: windows may be numerous; consumers must not assume a handful
+        // of long ones. Track-extent presence with a short re-acquisition
+        // timeout produces many short windows per actor.
+        var truth = new TruthFile();
+        for (var a = 0; a < 50; a++)
+        {
+            var actor = Actor();
+            for (var w = 0; w < 1000; w++)
+            {
+                actor.Scenes.Add([w * 10.0, (w * 10.0) + 4.0]);
+            }
+
+            truth.Actors.Add(actor);
+        }
+
+        var sw = Stopwatch.StartNew();
+        var present = PresenceLookup.ActorsPresentAt(truth, 5002.0).ToList();
+        sw.Stop();
+
+        // The response is bounded by actor count, never by window count — which
+        // is what keeps `jray?t=` small however finely presence is sliced.
+        Assert.Equal(50, present.Count);
+
+        // 50 000 windows scanned. Generous bound: this asserts the read path is
+        // not accidentally quadratic, not a precise budget on a shared runner.
+        Assert.True(sw.ElapsedMilliseconds < 250, $"lookup took {sw.ElapsedMilliseconds} ms");
+    }
+}