fix(physics): guard the world-frame agreement proven unreachable by measurement

Closes #283 (plan S3) - as UNREACHABLE, not by restructuring ownership.

acdream has two owners that convert a landblock-local network origin into the
streamed world frame: LiveWorldOriginState for presentation/streaming, and
RuntimePhysicsState.TryGetWorldFrameOffset for placement. They rebase on
different edges - Runtime the instant an accepted Position carries
TeleportAdvanced, App only once StreamingOriginRecenterCoordinator observes
old-window retirement completion, many frames later. A one-landblock
disagreement places an entity 192 m from the geometry around it: the same
failure family as the zero-offset bug 670f307c fixed, with a wrong origin
instead of a missing one.

The plan's first step was to prove or disprove reachability BEFORE moving
ownership, because a restructure on a hypothesis is churn. The probe added in
898ff18b answered it: a connected Release session recorded ZERO disagreements
across 11 completed reveals and six destination landblocks (0x0904, 0x1134,
0x3032, 0x8763, 0xA9B4, 0xF682) spanning roughly 45 km. A gap of even one
frame would have printed an offset in the tens of thousands of metres.

Cause of the safety: BeginOriginRecenter detaches EVERY resident landblock
before the new origin is adopted, so the two rebases are serialized and no
conversion can observe the gap. Ownership is therefore left exactly as it is.

What lands instead is the invariant that keeps it true.
LiveWorldOriginState.EnsureAgreesWithRuntimeFrame is checked at the
landblock->world conversion and is terminal on disagreement, converting a
silent 192 m-multiple misplacement into a loud failure with the offset in
metres and the landblock being projected. Six focused tests pin it, including
the cross-world portal case (0x09 -> 0xF6 = 45,504 m). Disagreement can no
longer reach the probe, so ACDREAM_PROBE_WORLD_FRAME now emits a verbose
per-conversion agreement trace - useful when a placement looks displaced for
some reason OTHER than a frame disagreement.

Complete Release solution: 10,844 passed / 4 skipped / 0 failed.

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

src/AcDream.Core/Physics/PhysicsDiagnostics.cs

@@ -71,12 +71,19 @@ public static class PhysicsDiagnostics
     /// anything converted in that gap lands a multiple of 192 m from the
     /// geometry App is building.
     ///
-    /// <para>When true, every projected placement compares Runtime's frame
-    /// centre against App's origin centre and emits one
-    /// <c>[world-frame]</c> line per DISAGREEMENT only — silence means the
-    /// window is never observed. This exists to prove or disprove
-    /// reachability before any ownership is restructured; it is a
-    /// measurement, never a gate. Initial state from
+    /// <para>A 2026-08-03 connected run answered that question: 11 reveals
+    /// across six landblocks spanning ~45 km recorded ZERO disagreements,
+    /// because the recenter detaches every resident landblock before adopting
+    /// the new origin and so serializes the two rebases. Disagreement is now
+    /// a terminal invariant
+    /// (<c>LiveWorldOriginState.EnsureAgreesWithRuntimeFrame</c>) rather than
+    /// something to observe.</para>
+    ///
+    /// <para>When true, this now emits one verbose <c>[world-frame] agree</c>
+    /// line per projected conversion recording the centre both owners used —
+    /// what you want when investigating a placement that looks displaced but
+    /// is NOT a frame disagreement. Measurement only; it never gates
+    /// placement. Initial state from
     /// <c>ACDREAM_PROBE_WORLD_FRAME=1</c>.</para>
     /// </summary>
     public static bool ProbeWorldFrameEnabled { get; set; } =