Add export summaries for archived cooks

[jongio]

ThermoWorks can export raw archive readings, but post-cook analysis still requires a spreadsheet. Add a summary export mode that reports useful per-channel stats for the selected archive.

Acceptance criteria:

• thermoworks export SERIAL --format summary prints per-channel count, min, max, average, first/last timestamp, and duration.
• --json returns the same summary data as structured JSON.
• The new format works with --archive, --output, and --downsample.
• Tests cover parsing and summary calculations.