feat: match guards, HOF inference, ownership C.2/C.3, LSP sema hover

Sessions 18–23 quality work:

- B.3c match arm guards + sequential found-flag lower (bootstrap + selfhost)
- Generic HOF type inference (Array/Iter map/filter/fold without type args)
- Pattern binding shadowing via unique C locals (__pN_src)
- Ownership C.2 exclusive &mut data-flow + C.4 goldens; *p= store-through fix
- Ownership C.3 auto-drop on early return/branches: scoped defers, move-on-return,
  Drop monomorphization, materialize return before Drop
- LSP 0.3.0: hover from real sema types
- Examples and QUALITY_PLAN session log; selfhost-loop identical

bootstrap/hir.nim

@@ -163,6 +163,7 @@ type
   HirMatchArm* = object
     pattern*: Pattern
     body*: HirNode
+    guard*: HirNode  ## optional: lowered `if cond` from `p if cond => body`
 
   HirFunc* = object
     name*: string