probe: ACDREAM_PROBE_REVEAL_TIMING — wall-clock attribution of the login/portal hold

[reveal-timing] lines from the reveal coordinator: per-dimension
first-ready edges (render neighborhood, composite textures, collision,
gate, materialization), 1 Hz progress with the resident-landblock count
(new GpuWorldState.LoadedLandblockCount), and one SUMMARY line at the
viewport reveal. Measurement-first groundwork for the login-load speedup:
the readiness barrier observes its dimensions serially, so the edges give
each dimension's observed tail while the progress lines expose the
pacing shape (a budget-paced linear drip reads directly off the counts).
Probe-gated in StreamingDiagnostics per Code Structure Rules §5; no
behavior change, one branch per Evaluate poll when unset.

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

src/AcDream.App/Streaming/GpuWorldState.cs

@@ -198,6 +198,10 @@ public sealed class GpuWorldState : ILiveEntitySpatialQuery
     /// that need to enumerate entities before the landblock is dropped
     /// (e.g. unregistering dynamic lights on a RemoveLandblock).
     /// </summary>
+    /// <summary>Resident landblock count — reveal-timing probe progress
+    /// lines only (<see cref="RevealTimingProbe"/>).</summary>
+    public int LoadedLandblockCount => _loaded.Count;
+
     public bool TryGetLandblock(uint landblockId, out LoadedLandblock? lb)
     {
         if (_loaded.TryGetValue(landblockId, out var found))