hako(parser): add index operator support

- ParserExprBox: support postfix [expr] and lower to Method(recv, "get", [index])
- ParserStmtBox: support index assignment IDENT [expr] = expr → Expr(Method(recv, "set", [idx, val]))
- Note: lowering uses existing Method → MIR emit path (int args). Phase‑1 canaries use int indices/values.

lang/src/compiler/parser/expr/parser_expr_box.hako

@@ -168,6 +168,16 @@ static box ParserExprBox {
             k = ctx.skip_ws(src, k)
             if src.substring(k, k+1) == ")" { k = k + 1 }
             node = "{\"type\":\"Method\",\"recv\":" + node + ",\"method\":\"" + mname + "\",\"args\":" + args_json2 + "}"
+          } else if tch == "[" {
+            // Index access: node[ index ] → Method(recv=node, method="get", args=[index])
+            k = k + 1
+            k = ctx.skip_ws(src, k)
+            local idx_json = ctx.parse_expr2(src, k)
+            k = ctx.gpos_get()
+            k = ctx.skip_ws(src, k)
+            if src.substring(k, k+1) == "]" { k = k + 1 }
+            local args_idx = "[" + idx_json + "]"
+            node = "{\\\"type\\\":\\\"Method\\\",\\\"recv\\\":" + node + ",\\\"method\\\":\\\"get\\\",\\\"args\\\":" + args_idx + "}"
           } else {
             cont2 = 0
           }
@@ -352,4 +362,3 @@ static box ParserExprBox {
     return out + "@" + ctx.i2s(j)
   }
 }
-