feat: incremental AST cache to eliminate redundant AstEncoder work

.gitignore

@@ -69,3 +69,6 @@ venv.bak/
 target/
 .vscode/target/
 Cargo.lock
+
+# PySpector incremental AST cache (generated, never version-controlled)
+.pyspector_cache/

src/pyspector/_ast_encode.py

@@ -0,0 +1,55 @@
+"""
+Shared AST → JSON encoder for PySpector.
+
+Single source of truth for the JSON schema consumed by the Rust core.
+Imported by both ast_cache.py and cli.py to eliminate encoder drift.
+"""
+from __future__ import annotations
+
+import ast
+import json
+from typing import Any, Dict
+
+
+class AstEncoder(json.JSONEncoder):
+    """Serialize ast.AST nodes to the JSON schema expected by the Rust core."""
+
+    def default(self, node: Any) -> Any:
+        if isinstance(node, ast.AST):
+            out: Dict[str, Any] = {
+                "node_type": node.__class__.__name__,
+                "lineno": getattr(node, "lineno", -1),
+                "col_offset": getattr(node, "col_offset", -1),
+            }
+            child_nodes: Dict[str, Any] = {}
+            simple_fields: Dict[str, Any] = {}
+            for fname, value in ast.iter_fields(node):
+                if type(value) is list:
+                    if value and all(isinstance(n, ast.AST) for n in value):
+                        child_nodes[fname] = value
+                    else:
+                        simple_fields[fname] = str(value) if value else []
+                elif isinstance(value, ast.AST):
+                    child_nodes[fname] = [value]
+                else:
+                    if isinstance(value, bytes):
+                        simple_fields[fname] = value.decode("utf-8", errors="replace")
+                    elif isinstance(value, int) and value.bit_length() > 14000:
+                        simple_fields[fname] = 0
+                    elif isinstance(value, (int, float, str, bool)) or value is None:
+                        simple_fields[fname] = value
+                    else:
+                        simple_fields[fname] = str(value)
+            out["children"] = child_nodes
+            out["fields"] = simple_fields
+            return out
+        if isinstance(node, bytes):
+            return node.decode("utf-8", errors="replace")
+        if hasattr(node, "__dict__"):
+            return str(node)
+        return super().default(node)
+
+
+def encode_node(node: ast.AST) -> str:
+    """Serialize a single AST node to JSON."""
+    return json.dumps(node, cls=AstEncoder)