fix(common): honor DeepSeek Beijing weekend boundaries

[c8dhjp4tyv-bit]

Fixes #1090

Summary

• Export and directly test the Beijing weekend predicate at the Friday/Sunday UTC boundary.
• Apply DeepSeek's weekend off-peak rule only from its effective instant, 2026-08-22T16:00:00Z.
• Keep historical pricing and expensive-window checks from retroactively treating pre-change weekends as off-peak.

Why

The existing weekend tests used instants where UTC and Beijing were already on the same weekend day, so removing the +8 hour conversion left the suite green. The new boundary cases pin Friday 16:00Z and Sunday 16:00Z, where the calendars diverge.

Verification

• bun test common/src/__tests__/freebuff-peak-hours.test.ts — 26 passed
• bun run --cwd common typecheck — passed
• Prettier check and git diff --check — passed

The full common test command cannot pass in this public checkout because several existing tests reference private/web files absent from the mirror; the changed test file passes independently.