refactor(input): own gameplay action routing

Move the sole semantic action-priority graph, combat and diagnostic commands, and retained-root item-drop lifetime behind focused typed owners. Preserve retail toggle behavior, explicit auto-wield cancellation, shutdown quiescence, and symmetric callback cleanup.

Co-authored-by: Codex <codex@openai.com>

docs/research/2026-07-11-combat-default-and-parent-event-pseudocode.md

@@ -3,6 +3,8 @@
 ## Sources
 
 - `ClientCombatSystem::GetDefaultCombatMode @ 0x0056B310`
+- `ClientCombatSystem::SetCombatMode @ 0x0056BE30`
+- `ClientCombatSystem::ToggleCombatMode @ 0x0056C8C0`
 - `ACCWeenieObject::GetObjectAtLocation @ 0x0058CE00`
 - `CM_Physics::DispatchSB_ParentEvent @ 0x006ACAF0`
 - `SmartBox::HandleParentEvent @ 0x004535D0`
@@ -52,10 +54,28 @@ GetDefaultCombatMode(showError):
         return Magic
 
     if showError == false:
-        print the retail cannot-use-held-item notice
+        print "You can't enter combat mode while wielding the %s"
+              formatted with held.GetObjectName(NAME_APPROPRIATE)
     return NonCombat
 ```
 
+`ToggleCombatMode` preserves that rejected result verbatim:
+
+```text
+ToggleCombatMode():
+    if current combat mode != NonCombat:
+        SetCombatMode(NonCombat, send = true)
+        return
+
+    requested = GetDefaultCombatMode(showError = false)
+    SetCombatMode(requested, send = true)
+```
+
+`SetCombatMode` immediately returns when `requested == current`. Therefore an
+incompatible Held object prints the notice in `GetDefaultCombatMode`, returns
+`NonCombat`, and produces no wire request or local mode transition. It is not
+coerced to unarmed Melee.
+
 `GetObjectAtLocation` walks the player's ordered inventory-placement list and
 returns the first entry whose location intersects the requested mask (and whose
 priority intersects the requested priority when that priority is non-zero).