A --since date later than --until is accepted and prints an empty report #1729

Summary

A window whose --since date is later than its --until date is not rejected. The report loads every entry, keeps nothing, and prints an empty table (or {"daily": [], "totals": {...}} with --json) with exit code 0. That looks like "no usage data" rather than a typo in the flags.

Reproduce

ccusage daily --since 2026-09-14 --until 2026-09-01
# empty table, exit 0
ccusage codex monthly --since 20260914 --until 20260901 --json
# {"monthly": [], "totals": {...}}, exit 0

Reproduced on main d341949 and on the published 20.0.20.

Expected

Reject the window before loading data, the same way #1604 rejects a bound that is not a real date:

The --since date '20260914' is later than the --until date '20260901'.
Run 'ccusage --help' for usage.

with exit code 2, for flags, config-file bounds, and a mix of both.

Notes

normalize_date_bound already stores both bounds as YYYYMMDD, so the check is a string comparison in ccusage-cli-parser after the command is parsed and config is applied. I have this implemented with tests and a docs line and will open the PR right after triage.