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

bootstrap/ast.nim

@@ -233,6 +233,9 @@ type
       exprClosureParams*: seq[Param]
       exprClosureBody*: Block
       exprClosureReturnType*: TypeExpr
+      captureCount*: int
+      captureNames*: seq[string]
+      captureTypeKinds*: seq[int]
 
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   # Statements