fix: total_members reports site membership as network users

[chubes4]

Problem

NetworkStats documents total_members as the total registered members across the network, but the provider switches to the Community blog and calls count_users(). In multisite, that counts users who are members of that site, not every row in the shared network user table.

Production evidence

On 2026-07-21:

• total_members returned 635.
• count_users() on Community returned 635.
• The shared users table contained 641 users.
• Six network users therefore disappear from a metric labeled and documented as network-wide.

TotalMembersProvider.php lines 5-9 explicitly assert that shared users make count_users() return the whole-network total; that assumption is false.

Expected behavior

Choose and enforce one honest contract:

• If the metric means all registered network users, use the canonical multisite network-user count and return 641 for this snapshot.
• If it intentionally means Community members, rename the key/label/provider so consumers do not present it as total network membership.

Acceptance criteria

• Add coverage with a network user who is not a Community-site member.
• Ensure the metric name, label, docs, and value agree.
• Keep the primitive in extrachill-network; consumers should not compensate for the mismatch.