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/cli/sql_cmd.py

@@ -102,8 +102,15 @@ def do_explain(cfg, sql: str):
     return explain(_ro_engine(cfg), sql, timeout_ms=cfg.execution.statement_timeout_ms)
 
 
-def do_run(cfg, sql: str, *, limit: int):
+def do_run(cfg, sql: str, *, limit: int, offset: int = 0):
     """Esecuzione controllata secondo il transport configurato (direct|rest)."""
+    from tht.execute.limit import inject_limit_offset
+
+    # Inject OFFSET (and an outer LIMIT) via subquery wrapping when offset > 0.
+    # When offset == 0 we let the inner runners apply LIMIT directly (existing path).
+    if offset:
+        sql = inject_limit_offset(sql, limit=limit, offset=offset)
+
     if cfg.database.transport == "rest":
         from tht.rest.execute import run_controlled_rest
 
@@ -157,15 +164,13 @@ def explain_cmd(
 def preview_cmd(
     file: Path = typer.Argument(...),
     limit: int = typer.Option(None, "--limit", help="Default: execution.max_preview_rows."),
+    offset: int = typer.Option(0, "--offset", help="Riga di partenza (0-based) per il paging."),
     session: str = typer.Option(None, "--session"),
+    json_out: bool = typer.Option(False, "--json", help="Output JSON puro per il backend (sopprime tabella rich)."),
     config: Path = CONFIG_OPT,
 ) -> None:
     """Esecuzione controllata con LIMIT iniettato; aggregati mostrati per interi."""
-    from rich.console import Console
-    from rich.table import Table
-
     from tht.execute import ExecutionError
-    from tht.execute.warnings import runtime_warnings, static_warnings
 
     cfg = _load_config_or_exit(config)
     require_action(cfg, "preview")
@@ -173,11 +178,30 @@ def preview_cmd(
     check = validate_or_exit(cfg, sql, session)
     effective_limit = limit if limit is not None else cfg.execution.max_preview_rows
     try:
-        result = do_run(cfg, sql, limit=effective_limit)
+        result = do_run(cfg, sql, limit=effective_limit, offset=offset)
     except ExecutionError as e:
         typer.secho(f"ERRORE: {e}", fg=typer.colors.RED, err=True)
         raise typer.Exit(code=1)
 
+    if json_out:
+        typer.echo(json.dumps(
+            {
+                "columns": result.columns,
+                "rows": [list(r) for r in result.rows],
+                "execution_ms": result.execution_ms,
+                "truncated": result.truncated,
+                "limit": effective_limit,
+                "offset": offset,
+            },
+            ensure_ascii=False,
+        ))
+        return
+
+    from rich.console import Console
+    from rich.table import Table
+
+    from tht.execute.warnings import runtime_warnings, static_warnings
+
     cells = len(result.rows) * len(result.columns)
     is_aggregate = (
         "aggregate" in cfg.execution.allow