closures with captures: env struct + global instance + body rewriting

- AST: captureCount + captureName0..7 + captureType0..7 in Expr
- Sema: Scope_LookupUpTo + closureScope for capture analysis
- HIR Lower: env struct generation, capture assignments in skLet,
  body rewriting (ekIdent -> hFieldAccess on env instance)
- C Backend: env struct def + global instance emission for closures
- Bootstrap: full capture support in sema, hir_lower, lir_lower, lir_c_backend
- Selfhost loop remains deterministic

src/hir.bux

@@ -18,8 +18,9 @@ const hAlloca: int = 12;
 const hLoad: int = 13;
 const hStore: int = 14;
 const hFieldPtr: int = 15;
-const hArrowField: int = 16;
-const hIndexPtr: int = 17;
+const hFieldAccess: int = 16;
+const hArrowField: int = 17;
+const hIndexPtr: int = 18;
 const hCall: int = 18;
 const hCallIndirect: int = 19;
 const hCast: int = 20;
@@ -93,6 +94,26 @@ struct HirFunc {
     retTypeName: String;
     body: *HirNode;
     isPublic: bool;
+    // Closure capture metadata
+    captureCount: int;
+    captureName0: String;
+    captureName1: String;
+    captureName2: String;
+    captureName3: String;
+    captureName4: String;
+    captureName5: String;
+    captureName6: String;
+    captureName7: String;
+    captureType0: int;
+    captureType1: int;
+    captureType2: int;
+    captureType3: int;
+    captureType4: int;
+    captureType5: int;
+    captureType6: int;
+    captureType7: int;
+    envStructName: String;
+    envInstanceName: String;
 }
 
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