feat(harness): tht sql preview --json + --offset for AGGrid paging (BE-2)

Add inject_limit_offset (tht/execute/limit.py) — pure subquery wrapper that
applies LIMIT/OFFSET non-destructively without clobbering user-supplied LIMITs.
Wire offset param into do_run (pre-processing when offset>0) and add --offset /
--json flags to preview_cmd; JSON mode emits pristine stdout with columns, rows,
execution_ms, truncated, limit, offset. 5 new tests (4 unit + 1 JSON-purity),
219/219 total passing (no regressions).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

harness/tht/execute/limit.py

@@ -0,0 +1,21 @@
+"""Pure subquery LIMIT/OFFSET injection for paging (non-destructive).
+
+Unlike `_inject_limit` (which uses AST rewriting and adds +1 for truncation detection),
+`inject_limit_offset` wraps the query as a subquery and applies LIMIT/OFFSET verbatim.
+This preserves any existing LIMIT that was part of the user's query (it becomes part
+of the inner subquery), and adds paging on top.
+"""
+
+
+def inject_limit_offset(sql: str, *, limit: int, offset: int = 0) -> str:
+    """Wrap query as subquery and apply LIMIT/OFFSET non-destructively.
+
+    The original query (including any user LIMIT) becomes the inner subquery,
+    so no existing LIMIT clause is overwritten. OFFSET clause is omitted when
+    offset == 0 to keep the SQL clean.
+    """
+    inner = sql.strip().rstrip(";")
+    clause = f"LIMIT {int(limit)}"
+    if offset:
+        clause += f" OFFSET {int(offset)}"
+    return f"SELECT * FROM (\n{inner}\n) AS _tht_page {clause}"