fix(extract): filter Kotlin builtin/stdlib types from references graph

_kotlin_collect_type_refs had no equivalent of _JAVA_BUILTIN_TYPES /
_PYTHON_ANNOTATION_NOISE / _GO_PREDECLARED_TYPES, so every Boolean/String/
Int/List/... type annotation in a .kt file became a real graph node/edge.
Unrelated files sharing only these language-level types were getting
merged into the same community by cluster-only.

Add _KOTLIN_BUILTIN_TYPES (kotlin.* scalars, collections, throwables -
deliberately not Android/JVM framework types like Context/View/Bundle,
matching how _JAVA_BUILTIN_TYPES doesn't filter framework types either)
and check it plus _JAVA_BUILTIN_TYPES (Kotlin freely references
java.util.Date/UUID/etc) at the three call sites that previously appended
unconditionally.

Deliberately excludes "Result" from the filter list even though
kotlin.Result exists - it collides with the very common user-defined
sealed-class name (confirmed against this repo's own sample.kt fixture,
which defines its own Result<T>).

Verified on a ~400-file Android/Kotlin project: 3693->3239 nodes (-12%),
6481->5221 edges (-19%), with a previously merged 5-file grab-bag
community cleanly splitting out an unrelated screen-dimension utility.

graphify/extractors/engine.py

@@ -607,6 +607,35 @@ def _php_method_return_type_node(method_node):
                 return c
     return None
 
+# Kotlin stdlib scalar/collection/core types that appear constantly as type
+# annotations but carry no useful semantic meaning as graph nodes (mirrors
+# _JAVA_BUILTIN_TYPES / _PYTHON_ANNOTATION_NOISE / _GO_PREDECLARED_TYPES).
+# Kotlin compiles to the JVM and freely references java.* types too, so this
+# is combined with _JAVA_BUILTIN_TYPES at the call site rather than duplicated.
+_KOTLIN_BUILTIN_TYPES = frozenset({
+    # kotlin — scalars & core
+    "Any", "Unit", "Nothing", "Boolean", "Byte", "Short", "Int", "Long",
+    "Float", "Double", "Char", "String", "CharSequence", "Number",
+    "Comparable", "Enum", "Annotation", "Pair", "Triple", "Lazy",
+    "Function",
+    # kotlin — throwables
+    "Throwable", "Exception", "RuntimeException", "Error",
+    "IllegalArgumentException", "IllegalStateException", "NullPointerException",
+    "IndexOutOfBoundsException", "ClassCastException", "NumberFormatException",
+    "ArithmeticException", "UnsupportedOperationException",
+    "NoSuchElementException", "ConcurrentModificationException",
+    "StackOverflowError", "OutOfMemoryError", "AssertionError",
+    "InterruptedException",
+    # kotlin.collections
+    "Array", "List", "MutableList", "ArrayList", "Set", "MutableSet",
+    "HashSet", "LinkedHashSet", "Map", "MutableMap", "HashMap",
+    "LinkedHashMap", "Collection", "MutableCollection", "Iterable",
+    "MutableIterable", "Iterator", "MutableIterator", "ListIterator",
+    "MutableListIterator", "Sequence", "Comparator",
+    # kotlin.text
+    "Regex", "MatchResult", "StringBuilder",
+})
+
 def _kotlin_user_type_name(user_type_node, source: bytes) -> str | None:
     """Return the head identifier text from a Kotlin user_type node (without generics)."""
     if user_type_node is None:
@@ -636,14 +665,14 @@ def _kotlin_collect_type_refs(node, source: bytes, generic: bool, out: list[tupl
         for c in node.children:
             if c.type in ("identifier", "type_identifier"):
                 text = _read_text(c, source)
-                if text:
+                if text and text not in _KOTLIN_BUILTIN_TYPES and text not in _JAVA_BUILTIN_TYPES:
                     out.append((text, "generic_arg" if generic else "type"))
                 break
             if c.type == "simple_user_type":
                 for sub in c.children:
                     if sub.type in ("identifier", "type_identifier"):
                         text = _read_text(sub, source)
-                        if text:
+                        if text and text not in _KOTLIN_BUILTIN_TYPES and text not in _JAVA_BUILTIN_TYPES:
                             out.append((text, "generic_arg" if generic else "type"))
                         break
                 break
@@ -659,7 +688,7 @@ def _kotlin_collect_type_refs(node, source: bytes, generic: bool, out: list[tupl
         return
     if t in ("identifier", "type_identifier"):
         text = _read_text(node, source)
-        if text:
+        if text and text not in _KOTLIN_BUILTIN_TYPES and text not in _JAVA_BUILTIN_TYPES:
             out.append((text, "generic_arg" if generic else "type"))
         return
     if t in ("nullable_type", "parenthesized_type", "type_reference"):

tests/test_languages.py

@@ -656,6 +656,27 @@ def test_kotlin_parameter_return_generic_and_field_contexts():
     assert ("run", "DataProcessor") in _edge_labels(r, "references", "generic_arg")
     assert ("DataProcessor", "Result") in _edge_labels(r, "references", "field")
 
+def test_kotlin_builtin_types_not_emitted_as_references():
+    # kotlin.* scalar/collection/core types used as parameter, return, or field
+    # types carry no useful graph meaning: they never resolve to a project node,
+    # so emitting `references` edges to them is pure noise (mirrors the Java
+    # _JAVA_BUILTIN_TYPES / Python _PYTHON_ANNOTATION_NOISE handling).
+    r = extract_kotlin(FIXTURES / "sample.kt")
+    ref_targets = {target for (_, target) in _edge_labels(r, "references")}
+    for builtin in ("String", "Int"):
+        assert builtin not in ref_targets, (
+            f"builtin type {builtin!r} should not be a references target"
+        )
+
+def test_kotlin_user_types_still_emit_references():
+    # Guard against over-filtering: a user-defined class sharing its name with a
+    # common domain-modeling identifier (Result) must still resolve to a real
+    # edge - the builtin filter is a fixed name list, so it must stay narrow
+    # enough not to swallow common user-chosen names like a sealed-class "Result".
+    r = extract_kotlin(FIXTURES / "sample.kt")
+    assert ("DataProcessor", "Result") in _edge_labels(r, "references", "field")
+    assert ("run", "DataProcessor") in _edge_labels(r, "references", "parameter_type")
+
 
 # ── Scala ─────────────────────────────────────────────────────────────────────