feat(def-navigation): surface last-loaded .def declaration for hover/GoTo

When a block template or address is declared in multiple PSC layers, hover and
Go to Definition now resolve to the effective declaration (the one from the
last-loaded .def file). Find References and Rename still include all
declarations.

- add effective_def_locations(documents, target, uris) helper (returns the last declaration)
- use effective_def_locations in lsp_server for goto/definition endpoints
- change def_hover_markdown to show the effective declaration and append a short
  "_Overrides ..._" note listing overridden declarations
- update changelog and add a test that verifies hover/GoTo reference the last
  loaded declaration while references include all declarations

server/src/tools/def_navigation.py

@@ -79,6 +79,13 @@ def def_definition_locations(
     ]
 
 
+def effective_def_locations(
+    documents: dict[str, DefDocument], target: DefTarget, uris: dict[str, str] | None = None
+) -> list[lsp.Location]:
+    """The declaration NX uses: ``documents`` follow the PSC layer order, the last one wins."""
+    return def_definition_locations(documents, target, uris)[-1:]
+
+
 def def_reference_locations(
     documents: dict[str, DefDocument],
     target: DefTarget,
@@ -170,6 +177,7 @@ def _address_table(declaration: DefDeclaration, formats: dict[str, DefDeclaratio
 
 
 def def_hover_markdown(documents: dict[str, DefDocument], target: DefTarget) -> str | None:
+    """Describe the effective (last loaded) declaration and name the ones it overrides."""
     declarations = def_declarations(documents, target)
     if not declarations:
         return None
@@ -180,12 +188,14 @@ def def_hover_markdown(documents: dict[str, DefDocument], target: DefTarget) ->
         if declaration.kind == FORMAT
     }
     kind, name = target
-    sections = []
-    for path, declaration in declarations:
-        header = f"**{_KIND_LABELS[kind]}** `{name}` — {Path(path).name}:{declaration.line + 1}"
-        if kind == ADDRESS:
-            body = _address_table(declaration, formats)
-        else:
-            body = f"```def\n{declaration.text}\n```"
-        sections.append(f"{header}\n\n{body}")
-    return "\n\n---\n\n".join(sections)
+    path, declaration = declarations[-1]
+    header = f"**{_KIND_LABELS[kind]}** `{name}` — {Path(path).name}:{declaration.line + 1}"
+    if kind == ADDRESS:
+        body = _address_table(declaration, formats)
+    else:
+        body = f"```def\n{declaration.text}\n```"
+    markdown = f"{header}\n\n{body}"
+    if len(declarations) > 1:
+        overridden = ", ".join(f"{Path(other).name}:{item.line + 1}" for other, item in declarations[:-1])
+        markdown += f"\n\n---\n\n_Overrides {overridden}_"
+    return markdown