fix(runtime): same-value LocalEntityId re-assertion is a no-op; file #365 headless hydration regression

The Campaign CH jump-probe headless reproduction quarantined on its first
advance tick: AdvanceBeforeNetwork re-asserts the resolved local entity id
every tick, which the C3c configuration seal treats as a mutation on a
dormant controller. A same-value write is now a no-op; a DIFFERENT id
while sealed still throws. One layer deeper the probe exposed #365: the
headless world never hydrates (entities stay 0, the movement controller
never publishes), so headless bots cannot move at head — filed with the
full evidence chain. HeadlessDiagnosticWriter.Failure now emits the full
exception detail (type-only cost a whole diagnosis round-trip).

Runtime tests 1,322/0, Headless tests 89/0.

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

src/AcDream.Headless/Diagnostics/HeadlessDiagnosticWriter.cs

@@ -58,6 +58,9 @@ internal sealed class HeadlessDiagnosticWriter
             sessionId,
             phase,
             errorType = error.GetType().FullName,
+            // Full detail: a quarantine that reports only the exception TYPE
+            // costs a whole diagnosis round-trip (jump-probe, 2026-08-10).
+            errorDetail = error.ToString(),
         });
     }
 

src/AcDream.Runtime/Gameplay/PlayerMovementController.cs

@@ -381,6 +381,14 @@ public sealed class PlayerMovementController
         get => _localEntityId;
         set
         {
+            // A same-value re-assertion is not a mutation:
+            // RuntimeLocalPlayerFrameController.AdvanceBeforeNetwork
+            // re-asserts the resolved id every advance tick, which on the
+            // headless host reached a still-dormant/sealed controller and
+            // quarantined the whole session (jump-probe repro, 2026-08-10).
+            // A DIFFERENT id while sealed remains the error it always was.
+            if (value == _localEntityId)
+                return;
             EnsureConfigurationMutable();
             _localEntityId = value;
         }