refactor(ast): cache source lines in transformer to avoid repeated file reads

Previously, `_build_callsite_metadata` opened and read the script file on every invocation,
which occurred frequently during atom lifting and top level function registration. This commit
moves the file read into the transformer's `__init__` method and stores the result in a
`self._source_lines` buffer.

Now, `_build_callsite_metadata` uses this in memory list to resolve the source line for a given
AST node, eliminating repeated disk I/O and improving performance.

Author: BloggerBust

preswald/engine/transformers/reactive_runtime.py

@@ -60,6 +60,7 @@ class AutoAtomTransformer(ast.NodeTransformer):
 
     def __init__(self, filename: str = "<script>"):
         self.filename = filename
+        self._source_lines = []
         self.current_function = None
         self.dependencies = {}
         self.known_components = self._discover_known_components()
@@ -77,6 +78,12 @@ def __init__(self, filename: str = "<script>"):
         self._module: ast.Module | None = None
         self._used_display_renderer_fns: set[str] = set()
 
+        try:
+            with open(filename, "r") as f:
+                self._source_lines = f.readlines()
+        except Exception as e:
+            logger.warning(f"[AST] Could not read source lines for file {filename}: {e}")
+
     @property
     def _current_frame(self) -> Frame:
         return self._frames[-1]
@@ -2694,13 +2701,9 @@ def _build_callsite_metadata(self, node: ast.AST, filename: str) -> dict:
         lineno = getattr(node, "lineno", None)
         source = ""
 
-        if filename and lineno:
-            try:
-                with open(filename, "r") as f:
-                    lines = f.readlines()
-                    source = lines[lineno - 1].strip() if 0 < lineno <= len(lines) else ""
-            except Exception:
-                pass
+        if lineno and self._source_lines:
+            if 0 < lineno <= len(self._source_lines):
+                source = self._source_lines[lineno - 1].strip()
 
         return {
             "callsite_filename": filename,