feat(physics): retail PvP exemption + viewer/creature/missile gates (Commit C)

The retail-faithful exemption block at the top of
CPhysicsObj::FindObjCollisions
(acclient_2013_pseudo_c.txt:276782-276839,276971), ported line-for-
line as a small static helper.

Behaviour now matches retail:
  - Two non-PK players walk through each other.
  - Two PK players collide.
  - Two PKLite players collide.
  - Mismatched PK status (PK vs non-PK, PK vs PKLite) — exempt.
  - Impenetrable target ("Free" PK status) — always collides.
  - Player vs creature/NPC — always collides (this is what closes the
    user-facing complaint that walking into a Holtburg vendor was
    walking through them).
  - Mover with IGNORE_CREATURES — walks through creature targets.
  - Viewer (camera ray) — walks through creatures.
  - Target with ETHEREAL+IGNORE_COLLISIONS — universally exempt.

CollisionExemption.ShouldSkip(targetState, targetFlags, moverState)
  - new file src/AcDream.Core/Physics/CollisionExemption.cs.
  - 13-test matrix covering every documented case
    (CollisionExemptionTests.cs).
  - Static + pure → cheap to call from the hot path.

Wiring:
  - TransitionTypes.FindObjCollisions: after broadphase distance
    reject, call ShouldSkip on the obj and ObjectInfo.State; on true,
    `continue`. Static landblock entries (State=0, Flags=None) fall
    through cheaply — no behavior change for static collision.
  - PhysicsEngine.ResolveWithTransition: new optional moverFlags
    parameter (default None for back-compat). PlayerMovementController
    passes ObjectInfoState.IsPlayer; remote dead-reckoning leaves it
    None (matches non-player movers, no PvP exemption applies).
  - PK/PKLite/Impenetrable bits for the LOCAL player are not yet
    sourced from PlayerDescription's PlayerKillerStatus property —
    that's a follow-up. Default "non-PK player" matches ACE's
    character-creation default and the user's +Acdream test
    character.

Cross-checked against ACE PhysicsObj.cs:381-405 (line-for-line C# port
of the same retail block). Only intentional divergence: ACE adds
state.HasFlag(IsImpenetrable) (mover-impenetrable) to the collide list;
retail's pseudo-C only checks the target — acdream follows retail.

dotnet build green, dotnet test 1467 passing (+13 new). Live test:
+Acdream walking into Holtburg vendors now stops at their cylinder;
walking through small plants still passes (Commit B's phantom skip).

Closes the live-entity collision arc: A (plumbing) + B (registration)
+ C (exemption).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -388,13 +388,21 @@ public sealed class PhysicsEngine
         float sphereRadius, float sphereHeight,
         float stepUpHeight, float stepDownHeight,
         bool isOnGround,
-        PhysicsBody? body = null)
+        PhysicsBody? body = null,
+        ObjectInfoState moverFlags = ObjectInfoState.None)
     {
         var transition = new Transition();
         transition.ObjectInfo.StepUpHeight = stepUpHeight;
         transition.ObjectInfo.StepDownHeight = stepDownHeight;
         transition.ObjectInfo.StepDown = true;
 
+        // Commit C 2026-04-29 — caller-supplied mover flags drive the
+        // retail PvP exemption block in FindObjCollisions. The local
+        // player passes IsPlayer (and PK/PKLite/Impenetrable when known
+        // from PlayerDescription); remote dead-reckoning passes None
+        // (matches non-player movement, all targets collide).
+        transition.ObjectInfo.State |= moverFlags;
+
         if (isOnGround)
             transition.ObjectInfo.State |= ObjectInfoState.Contact | ObjectInfoState.OnWalkable;