BMT 2025 archive: reorder rounds and split tiebreakers into their own table

[devin-ai-integration]

Summary

BMT 2025's archive now lists Power → General → Algebra → Calculus → Discrete → Geometry → Guts, with the five tiebreakers moved to a second table in the same subject order (rows keep their "… Tiebreaker" names).

The archive template gained an optional multi-table form; pages keep working unchanged with the old single extra.archive.tests list:

[[extra.archive.groups]]
label = "Round/Test"     # first column header, defaults to "Round/Test"
[[extra.archive.groups.tests]]
title = "Power"
problems = "power-problems.pdf"

The first column header is now "Round/Test" everywhere (it was "Round"), since the table markup is shared across all archive pages.

Link to Devin session: https://app.devin.ai/sessions/b2cad97fe85e4ad7aa2ed105e43fc39b
Requested by: @alphdelta

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.