Growth collector: funnel window and retention buckets use different 7-day spans

[lauurnce]

From the first real weekly growth report (VANTAGE, 2026-08-25).

What
The funnel table counts a trailing-7-days window (e.g. Aug 18–24) while retention buckets are Monday-anchored PH weeks (e.g. Aug 17–23). Adjacent but non-identical spans make cross-table reads subtly wrong.

Fix shape
Pick one convention for both (Monday-anchored PH week is the repo's dominant convention) and align scripts/reports/growth.ts windowing.