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/lir_c_backend.nim

@@ -626,6 +626,20 @@ proc emitModule*(be: var LirCBackend, builder: LirBuilder, module: HirModule): s
     be.emitLine("};")
     be.emitLine("")
 
+  # Emit env structs for closures with captures
+  for f in module.funcs:
+    if f.captureNames.len > 0 and f.envStructName != "":
+      be.emitLine(&"struct {f.envStructName} {{")
+      be.indent += 1
+      for i in 0 ..< f.captureNames.len:
+        let capName = f.captureNames[i]
+        let capType = if i < f.captureTypes.len: typeToCStr(f.captureTypes[i]) else: "int"
+        be.emitLine(&"{capType} {capName};")
+      be.indent -= 1
+      be.emitLine("};")
+      be.emitLine(&"static struct {f.envStructName} {f.envInstanceName};")
+      be.emitLine("")
+
   # Emit all LIR functions
   for f in builder.funcs:
     be.emitFunc(f, funcRetTypes, funcPtrTypes)