docs: refresh size, weight, and document-count claims from a clean-room audit

Measured by installing snapshot-20260611 (stable) and
snapshot-20260610-beta.3 into fresh data dirs, all three storage
profiles each, on an M2 Pro: archive 1.89/1.90 GB (was 1.62), profiles
~4.6/~7.1/~10.5 GB on disk (were ~3/~5.5/~8.6), 353,325 documents,
361,823 rendered files per format, semantic chunk index ~0.7 GB (was
~0.5), DB 5.2 -> 2.7 GB under compact. Also documents the fresh-install
beta-channel resolution rule.