test(common): pin the Beijing weekend shift with edge instants

[rekty]

{
"title": "test(common): pin the Beijing weekend shift with edge instants",
"head": "rekty:fix/pin-beijing-weekend-shift-tests",
"base": "main",
"body": "Closes #1090\n\n## Problem\n\nBoth weekend tests sit at 02:00Z, inside the band where the +8h Beijing shift and a plain getUTCDay() agree. Deleting the shift leaves deepseekPricingWindow returning identical answers for all 168 hours of a week — the suite stays green, and the file only starts lying the day DeepSeek moves a window past 16:00Z.\n\n## Fix\n\n1. Export isBeijingWeekend from freebuff-peak-hours.ts (doc-comment explains why: the weekend band Fri 16:00Z → Sun 16:00Z never overlaps the peak windows, so only the predicate can pin the shift directly).\n2. Add four edge-pinning tests at the exact instants where the two calendars disagree:\n\n| instant | Beijing | weekend |\n|---|---|---|\n| 2026-08-28T15:00:00Z | Fri 23:00 | no |\n| 2026-08-28T16:00:00Z | Sat 00:00 | yes |\n| 2026-08-30T15:00:00Z | Sun 23:00 | yes |\n| 2026-08-30T16:00:00Z | Mon 00:00 | no |\n\n## Verification\n\n- All 24 tests in the file pass with the fix in place.\n- Mutation check: removing the + 8 * 60 * 60 * 1000 shift makes exactly the two boundary tests fail (16:00Z Friday and 16:00Z Sunday), confirming the suite now catches the regression.\n"
}