fix(visitor): fold, don't drop, definitions past the per-line cap

_add capped each (file, line) bucket at 8 entries and dropped the rest.
When a type appears in many compile units (e.g. BlockType across ~160
CUs), only one copy carries a fully-defined nested type such as
BlockType::NameInfo, and it could fall past the cap and be discarded,
leaving the nested type referenced but never defined.

Now, once the bucket is full, fold the overflow object into an existing
entry it can merge with instead of dropping it, preserving the unique
content it carries. The append + parameter-name sync + merge flow is
otherwise untouched, so parameter names are unaffected.

Also guard TypeDef.merge against a None value, which the extra merges
can now reach.

Author: wu-vincent

src/dwarf2cpp/models.py

@@ -194,12 +194,13 @@ def merge(self, other: Object) -> bool:
         if self.kind != other.kind or self.name != other.name or type(self.value) is not type(other.value):
             return False
 
-        if isinstance(self.value, str):
+        if self.value is None:
+            pass
+        elif isinstance(self.value, str):
             if self.value != other.value:
                 return False
-        else:
-            if not self.value.merge(other.value):
-                return False
+        elif not self.value.merge(other.value):
+            return False
 
         self.alignment = self.alignment or other.alignment
         return True

src/dwarf2cpp/visitor.py

@@ -752,14 +752,15 @@ def generic_visit(self, die: DWARFDie) -> None:
             self.visit(child)
 
     def _add(self, filepath: str, lineno: int, obj: Object) -> None:
-        file = self._files[filepath]
-        lines = file[lineno]
+        lines = self._files[filepath][lineno]
 
-        if len(lines) >= 8:
-            # too many items on a single line (template instantiations?)
+        if len(lines) < 8:
+            lines.append(obj)
             return
 
-        lines.append(obj)
+        for existing in lines:
+            if existing.merge(obj):
+                return
 
     def _register_template(self, key: str | int, lineno: int, template: Template) -> Template | None:
         if not template.declaration: