Let a jsonl export keep a record a value

`--format jsonl` existed, but `_cell` ran json.dumps at row-build time,
before a format was chosen — so a nested value was a string by then and
jsonl only re-escaped it, leaving a consumer against csv's 128KB field
limit either way. Stringifying moved to the csv writer, so csv is
byte-identical and jsonl carries json. The runs TUI followed, or a record
would draw as a Python repr.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KYM38KSb4V4v2T71eifnZv

backend/fluksio/tui/compare.py

@@ -9,6 +9,7 @@ So nothing here computes what a route already knows.
 
 from __future__ import annotations
 
+import json
 from typing import Any
 
 from rich.text import Text
@@ -45,6 +46,10 @@ def _cell(value: Any) -> str:
     """
     if isinstance(value, float):
         return f"{value:.4g}"
+    if isinstance(value, (dict, list)):
+        # An export keeps a record a value now, and python's repr of one is
+        # not what anybody reading a run wrote down.
+        return json.dumps(value)
     return "" if value is None else str(value)