feat(selfhost): auto-drop for interface-based Drop implementations

- Extend Lcx_BuildAutoDropFree to detect TypeName_Drop methods

  registered by sema for extend Type for Drop impl blocks.

- Preserve existing @[Drop] attribute path and move semantics.

- Add design doc and implementation plan.

- Mark Destructors/Drop roadmap item done in selfhost.

docs/superpowers/specs/2026-06-10-drop-trait-design.md

@@ -1,7 +1,7 @@
 # Drop Trait / Destructors Design Document
 
 > **Date:** 2026-06-10  
-> **Status:** Approved  
+> **Status:** Superseded — see [2026-06-14-drop-interface-auto-drop-design.md](2026-06-14-drop-interface-auto-drop-design.md)  
 > **Scope:** Selfhost compiler (`src/*.bux`)
 
 ## 1. Problem Statement