fix(physics): P5 commit 3 - retire TS-35, close #167 (ConstraintManager leash)

PhysicsBody.IsFullyConstrained now reflects real ConstraintManager state
(pushed every tick by the same per-tick pumps commit 2 wired), so
jump_is_allowed's already-ported gate (WeenieError 0x47) actually fires
while an object is rubber-banding hard against a server position
correction, closing the last piece of #167.

Housekeeping:
- Delete register row TS-35 (retired: the write side is no longer stubbed).
- Rewrite the stale doc comments on PhysicsBody.IsFullyConstrained,
  ConstraintManager (class + IsFullyConstrained), PositionManager.ConstrainTo,
  EntityPhysicsHost.PositionManager, and PlayerMovementController.PositionManager
  that described the leash as permanently unarmed/stubbed.
- Close #167 in ISSUES.md citing the research doc and commits e0629145 /
  7719d25b.
- Add an "as-ported" addendum to
  docs/research/2026-07-30-constraint-leash-constants.md naming the actual
  current seam owners (the doc's own open question flagged this as
  implementer-verify-required post-J-slices).
- Update docs/plans/2026-07-29-physics-parity-campaign.md's P5 status and
  CLAUDE.md's Campaign P summary to reflect #167's closure (items #153/#72
  remain open in P5).

Verification: complete solution suite green - 9,978 tests, 5 skips, 0
failures across all 9 test projects (Core.Tests, Runtime.Tests, App.Tests,
Headless.Tests, Core.Net.Tests, Content.Tests, UI.Abstractions.Tests,
Bake.Tests, Cli.Tests).

src/AcDream.Core/Physics/PhysicsBody.cs

@@ -408,16 +408,33 @@ public sealed class PhysicsBody
     public double LastUpdateTime { get; set; }
 
     /// <summary>
-    /// R3-W3 stub for retail <c>CPhysicsObj::IsFullyConstrained</c>
-    /// (0x0050f730), read by <c>CMotionInterp::jump_is_allowed</c> (raw
-    /// 305524-305525: <c>if (IsFullyConstrained(physics_obj) != 0) return
-    /// 0x47;</c>). Retail's body walks per-cell contact-plane constraints
-    /// (a mover pinned between opposing walkable surfaces / doorway
-    /// jamming); acdream has no equivalent constraint-tracking yet.
-    /// Register row: stubbed false (never fires) — a real port needs the
-    /// per-cell shadow-list contact accounting the physics digest tracks.
-    /// See docs/architecture/retail-divergence-register.md (added same
-    /// commit as this field).
+    /// Retail <c>CPhysicsObj::IsFullyConstrained</c> (0x0050f730), read by
+    /// <c>CMotionInterp::jump_is_allowed</c> (raw 305524-305525:
+    /// <c>if (IsFullyConstrained(physics_obj) != 0) return 0x47;</c>) to block
+    /// a jump while the object is rubber-banding hard against a server
+    /// position correction.
+    ///
+    /// <para>R3-W3 originally stubbed this as an always-false property under
+    /// a WRONG mechanism guess (per-cell contact-plane / doorway-jamming).
+    /// R5-V1 corrected the mechanism: the real retail source is
+    /// <c>ConstraintManager::IsFullyConstrained</c> (0x005560d0,
+    /// <c>constraint_distance_max * 0.9 &lt; constraint_pos_offset</c>) —
+    /// <see cref="Motion.PositionManager"/>'s constraint sub-manager. R5-V1
+    /// ported <see cref="Motion.ConstraintManager"/> but did not arm it (#167,
+    /// former register row TS-35).</para>
+    ///
+    /// <para>Campaign P P5 (2026-07-30) armed the leash at every current
+    /// inbound-position acceptance seam
+    /// (<c>LiveEntityNetworkUpdateController</c> for remotes,
+    /// <c>PlayerMovementController.SetPosition</c>/<c>BlipPosition</c> for the
+    /// local player — see <c>docs/research/2026-07-30-constraint-leash-constants.md</c>
+    /// §2/§3). <see cref="MotionInterpreter"/> only holds a
+    /// <see cref="PhysicsBody"/> reference (no host), so this property stays a
+    /// plain settable bool; the per-tick pump that already runs
+    /// <c>PositionManager.AdjustOffset</c> (<c>PlayerMovementController.Update</c>,
+    /// <c>RuntimeRemotePhysicsUpdater.Tick</c>/<c>TickHidden</c>) is the single
+    /// owner that pushes <c>PositionManager.IsFullyConstrained()</c> here every
+    /// tick, so this read is now live, not stubbed.</para>
     /// </summary>
     public bool IsFullyConstrained { get; set; }