Harden NIP-29 discovery and local compose defaults

[mwd474747]

Summary:

• Makes channel reconcile require complete NIP-29 discovery heads before skipping: metadata, admins, and members.
• Preserves relay self p-tags with allow_self_tagging for discovery event emission.
• Pins local compose helper images and binds development surfaces to loopback by default.

Verification:

• git status -sb
• git status --porcelain
• git diff --check
• ./bin/cargo fmt --check
• ./bin/cargo test -p buzz-admin discovery_
• ./bin/cargo test -p buzz-relay discovery_completeness
• ./bin/cargo check -p buzz-admin
• ./bin/cargo check -p buzz-relay
• docker compose -f deploy/compose/compose.yml -f deploy/compose/compose.dev.yml config --quiet

Boundaries:

• Draft PR only; no merge or release.
• No Docker recreate, volume deletion, room mutation, public expose, LAN bind expansion, signed installer, or secret material in receipts.