feat: read measurements at a pinned commit

measurements-pin.json names the measurements repository and the commit read. sync_measurements.py reads the clone in .cache/measurements at that commit, checks every line against its id and sidecar, and writes the site index and the SQLite databases. --fetch refuses a remote that changed or removed a line the pin holds, --bump moves the pin. validate_schemas checks every line and sidecar against their schemas, check_freshness flags a pin behind the remote head.

scripts/artifacts.py

@@ -8,6 +8,7 @@ from __future__ import annotations
 
 from collections.abc import Callable, Iterator
 import contextlib
+import json
 import os
 import tempfile
 import re
@@ -24,6 +25,15 @@ _TIMESTAMP_PATTERNS = [
     re.compile(r'<time datetime="[^"]*">Data as of [^<]*</time>'),
 ]
 
+def json_text(value: object) -> str:
+    """Stable scalar representation for CSV and SQLite exports."""
+    if value is None:
+        return ""
+    if isinstance(value, (dict, list, tuple, bool)):
+        return json.dumps(value, ensure_ascii=False, sort_keys=True)
+    return str(value)
+
+
 def write_if_changed(
     path: str, content: str, normalize: Callable[[str], str] | None = None
 ) -> bool: