feat(harness): persist CTE preview rows, add tht cte info, --doc for cte plan

WS1 of review-gates-v2: gives the JS gate (WS2) deterministic data to build the
cte_result v2 payload.

- CteTestRecord gains optional preview_rows (JSON-coerced, truncated cells);
  test_cmd populates it from the bounded result rows.
- New read-only `tht cte info <name> --session <id> [--json]`: plan
  index/total, persisted .sql, cte_plan_doc.json entry (if any), last
  CteTestRecord. Exits 1 with a clean stderr message on missing
  session/plan/name/sql.
- `tht cte plan --doc -` validates a chain-doc JSON (ctes[].name must match
  --name, same order) and writes it to cte_plan_doc.json; cte_plan.json stays
  a plain list[str] (load-bearing for tht.phase.next_cte). --doc is optional.

harness/tests/test_ctetest.py

@@ -7,7 +7,8 @@ and the ledger I/O (load/append, tolerant of JSON-array and JSONL formats).
 Pure logic, no DB.
 """
 import json
-from datetime import datetime
+from datetime import date, datetime
+from decimal import Decimal
 from pathlib import Path
 
 import pytest
@@ -15,6 +16,7 @@ import pytest
 from tht.ctetest import (
     CteError,
     CteTestRecord,
+    _jsonable,
     append_cte_test,
     build_test_sql,
     has_trailing_select,
@@ -131,3 +133,60 @@ def test_corrupt_ledger_raises(tmp_path):
     (tmp_path / "cte_tests.json").write_text("{not valid json")
     with pytest.raises(CteError, match="corrotto"):
         load_cte_tests(tmp_path)
+
+
+# --- _jsonable ----------------------------------------------------------
+
+def test_jsonable_passes_through_native_types():
+    assert _jsonable(None) is None
+    assert _jsonable(True) is True
+    assert _jsonable(1) == 1
+    assert _jsonable(1.5) == 1.5
+    assert _jsonable("abc") == "abc"
+
+
+def test_jsonable_coerces_decimal_and_date_to_str():
+    assert _jsonable(Decimal("12.34")) == "12.34"
+    assert _jsonable(date(2025, 1, 1)) == "2025-01-01"
+    assert _jsonable(datetime(2025, 1, 1, 12, 0, 0)) == "2025-01-01 12:00:00"
+
+
+def test_jsonable_truncates_long_strings():
+    long_str = "x" * 500
+    out = _jsonable(long_str)
+    assert isinstance(out, str)
+    assert len(out) <= 200
+
+
+def test_jsonable_truncates_long_coerced_values():
+    class Weird:
+        def __str__(self):
+            return "y" * 500
+
+    out = _jsonable(Weird())
+    assert len(out) <= 200
+
+
+# --- preview_rows on CteTestRecord ---------------------------------------
+
+def test_preview_rows_optional_default_none():
+    r = _record()
+    assert r.preview_rows is None
+
+
+def test_preview_rows_roundtrip_through_ledger(tmp_path):
+    rec = _record(name="a", preview_rows=[["x", 1], [None, 2.5]])
+    append_cte_test(tmp_path, rec)
+    loaded = load_cte_tests(tmp_path)
+    assert loaded[0].preview_rows == [["x", 1], [None, 2.5]]
+
+
+def test_old_record_without_preview_rows_still_validates(tmp_path):
+    # back-compat: an old cte_tests.json row has no "preview_rows" key at all
+    path = tmp_path / "cte_tests.json"
+    path.write_text(json.dumps({
+        "name": "x", "ts": "2025-01-01T00:00:00", "sql_hash": "h", "status": "ok",
+    }))
+    loaded = load_cte_tests(tmp_path)
+    assert len(loaded) == 1
+    assert loaded[0].preview_rows is None