fix: keep decompose distinct from compose

The grammar captures both directives under :compose; the builder
dropped the value and compiled every decompose as NFC compose —
var-kor's author-intended NFD output composed o+U+0306 into ŏ
(U+014F) and all 94 of its sweep tests failed on invisible byte
diffs. Read the captured value.

var-kor: 282 examples, 0 failures (was 94).

Author: ronaldtse

lib/interscript/isc/document_builder.rb

@@ -263,7 +263,10 @@ def extract_stage_items(n)
         elsif n[:separate]
           [{kind: :separate, separator: n[:separator] ? materialize(n[:separator]) : nil}]
         elsif n[:compose]
-          [{kind: :compose}]
+          # The grammar captures both `compose` and `decompose` under this
+          # key — the value decides the directive. Dropping it compiled
+          # every decompose as NFC compose.
+          [{kind: n[:compose].to_s.to_sym}]
         elsif n[:case]
           item = {kind: :string_case, op: n[:case].to_s}
           item[:kwargs] = extract_kwargs(n[:case_kwargs]) if n[:case_kwargs]

spec/interscript/isc/node_adapter_spec.rb

@@ -373,3 +373,25 @@ def parse_and_adapt(src)
     expect(node.name).to eq("test-map")
   end
 end
+
+RSpec.describe "NodeAdapter compose vs decompose" do
+  it "keeps decompose distinct from compose" do
+    src = <<~ISC
+      system "t:a-b:C-D:1" {
+        metadata { name "T" }
+        stage main {
+          sub "a" "b"
+          decompose
+          compose
+        }
+      }
+    ISC
+    node = Interscript::Isc::NodeAdapter.to_interscript_node(
+      Interscript::Isc::DocumentBuilder.build(Interscript::Isc::Parser.parse(src))
+    )
+    calls = node.stages[:main].children.select { |c| c.is_a?(Interscript::Node::Rule::Funcall) }.map(&:name)
+    # The grammar captures both directives under :compose; the builder must
+    # read the value or decompose compiles as NFC compose.
+    expect(calls).to eq(%i[decompose compose])
+  end
+end