test(stand): assert the export footer split instead of stepping around it

[ktursunov]

The Git output table renders a Total row and a Grand total row under its
series. The workbook export reproduces both; the CSV stops at the last bucket —
on purpose, because its consumers parse every row as data (24fe8baf fix(metrics): omit CSV footer rows, locked by metric-timeseries-export.test.ts).

The stand journey compared bucket rows only. That sidestepped the contract: it
proved neither half, and either format could drift across it unnoticed.

What the journey asserts now

CSV	carries no Total or Grand total row
XLSX	its Total row equals the one the table renders
XLSX	its grand totals are the numbers the table shows

Still nothing hand-written: the expectation is the rendered page. The table puts
lines added and removed in one signed cell where the export gives them a column
each, so the numbers are compared rather than the strings.

Verification

The assertions were run against output captured verbatim from a seeded stand —
the rendered table, the CSV and the workbook — and pass. Six mutations of that
output each fail the assertion that covers them: a Total footer appearing in
the CSV, a Grand total footer appearing in the CSV, the workbook's Total
drifting, the workbook losing its Total row, its grand totals drifting, and a
lines column going missing.

The end-to-end run is this PR's ui-journeys lane: the stand I had captured
from was torn down before the change was written, so the browser leg is verified
in CI rather than locally.

[coderabbitai]

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