fix(combat): escape occupied login positions

Port retail's radius-aware placement ring so a relogging player is seated beside creatures occupying the saved location, and register the local body in the shared resolved-shadow pipeline. Route new forward movement and jump through AbortAutomaticAttack so repeat combat cancels immediately on movement.

Co-Authored-By: Codex <codex@openai.com>

docs/research/2026-07-12-login-placement-and-repeat-cancel-pseudocode.md

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+# Login placement and repeat-attack cancellation — retail pseudocode
+
+## Sources
+
+- `CTransition::find_placement_pos` @ `0x0050BA50`
+- `CTransition::find_placement_position` @ `0x0050C170`
+- `CObjCell::find_obj_collisions` @ `0x0052B750`
+- `CPhysicsObj::add_shadows_to_cells` @ `0x00514AE0`
+- `CPhysicsObj::enter_world` @ `0x00516170`
+- `ClientCombatSystem::AbortAutomaticAttack` @ `0x0056AE90`
+- `ClientCombatSystem::CommenceJump` @ `0x0056AF90`
+- `ACCmdInterp::HandleNewForwardMovement` @ `0x0058B1F0`
+- Named pseudo-C: `docs/research/named-retail/acclient_2013_pseudo_c.txt`
+- Interpretation cross-check: ACE `Transition.cs`, `CommandInterpreter.cs`, and
+  `ACCmdInterp.cs` on the `ACEmulator/ACE` `master` branch.
+
+## Enter-world placement
+
+Retail `enter_world(position)` stores the server position and calls
+`enter_world(true)`. That constructs `SetPosition` flags `0x11`, so placement
+may slide away from an obstruction.
+
+```text
+find_placement_position():
+    check = current
+    insert_type = INITIAL_PLACEMENT
+    test environment/cells at check
+      (CObjCell skips shadow-object collisions for INITIAL_PLACEMENT)
+    validate initial placement
+
+    insert_type = PLACEMENT
+    if !find_placement_pos(): fail
+
+    optionally step down to a walkable surface
+    validate final placement
+```
+
+`find_placement_pos` is the object-aware nearest-clear-point search:
+
+```text
+find_placement_pos():
+    check = current
+    clear sliding/contact state
+    if placement_test(check) succeeds:
+        accept check and return true
+
+    if placement sliding is disabled:
+        return false
+
+    search_distance = 4 metres
+    search_radius = 4 metres
+    radius = mover foot-sphere radius
+
+    if radius < 0.125 metres:
+        fake_sphere = true
+        search_radius = 2 metres
+    else if radius < 0.48 metres:
+        radius = 0.48 metres
+
+    exact_step_count = 4 / radius
+    if fake_sphere: exact_step_count *= 0.5
+    if exact_step_count <= 1: return false
+
+    step_count = ceil(exact_step_count)
+    distance_step = search_radius / step_count
+    radians_step = PI * distance_step / radius
+    total_distance = 0
+    total_radians = 0
+
+    for each radial step:
+        total_distance += distance_step
+        total_radians += radians_step
+        compass_samples = ceil(total_radians) * 2
+        heading_step_degrees = 360 / compass_samples
+
+        for sample = 0 .. compass_samples-1:
+            check = current
+            heading = heading_step_degrees * sample
+            offset = forward_vector(heading) * total_distance
+            offset = adjust_offset(offset)
+            if length(offset) < epsilon: continue
+
+            check += offset
+            clear sliding/contact state
+            if placement_test(check) succeeds:
+                accept check and return true
+
+    return false
+```
+
+The local player must also own a normal shadow entry. Retail registers every
+placed `CPhysicsObj`; collision queries exclude only the moving object's own
+pointer. Therefore remote monsters can collide/de-overlap against the local
+player, while the player's placement search skips its own shadow by entity id.
+
+## Movement cancels repeat attack
+
+```text
+ACCmdInterp.HandleNewForwardMovement():
+    if combat_system exists:
+        combat_system.AbortAutomaticAttack()
+    base.HandleNewForwardMovement()  // also stops auto-run
+
+ClientCombatSystem.AbortAutomaticAttack():
+    if server_response_pending
+       or attack_request_in_progress
+       or attack_in_progress
+       or repeat_attacking:
+        send Event_CancelAttack
+        repeat_attacking = false
+        if combat power build is active:
+            HidePowerBar()
+```
+
+`CommandInterpreter::AddCommand` calls `HandleNewForwardMovement` for the
+forward-substate command list, which includes forward/backward/autorun rather
+than the independent turn and sidestep lists. Jump has the same cancellation
+at `ClientCombatSystem::CommenceJump` before its jump power bar begins.
+