docs(specs): spec S17 phase 8 statistics

Why

Phase 8 is S17's statistics surface: the eight ADR-009 GET routes serving storage and download figures at namespace, format, repository, and artifact scope. The monolith disable confirmation needs the namespace figures (repository count and stored size), and S22's implementation will land the columns the namespace endpoint reads. The patch specs all eight routes now; implementation follows S22's schema migration, and S18 is deliberately not a gate, so per-artifact download counts join later as additive fields.

The review decisions live in Resolutions. The load-bearing ones: per-artifact byte footprints are derived exactly at read time rather than adding the unowned ADR-007 size columns, live sizes include soft-deleted-but-stored rows while live entity counts are visible-only, and statistics on virtual and remote repositories stay owned by this phase as an explicit follow-up rather than deferring silently to Phase 6, which docs(specs): spec S17 phase 6 virtual and remot... (!1408 - merged) • Hayley Swimelar • 19.3 does not cover.

Note: this MR and the Phase 6 spec MR patch the same document regions (summary phase list, phase overview, Resolutions); whichever merges second rebases mechanically.

Test plan

Docs-only. markdownlint and Vale pass locally with zero errors; the docs lint CI jobs are the gate.

Related to S17 Phase 8: statistics (#316 - closed) • Hayley Swimelar