Allow a one-member sharded cluster to own regions

[austin-barrington]

Summary

• Bootstrap now uses RF = min(configured, live members), so a 1-node cluster with sharding on owns its first region (peers=[self], primary=self) even when configured RF is 3.
• Adds one_member_cluster_owns_region_after_first_write (P1.1) plus unit coverage for the effective-RF helper.

Server defaults stay off. Operator follow-on (webhook + [cluster] enabled=true at replicas=1 with explicit sharding) is in hyperbytedb-operator.

Test plan

• cargo test --lib (includes effective_rf_never_exceeds_membership, bootstrap_n1_peers_are_self)
• cargo test --test sharding_cluster_integration one_member_cluster_owns_region_after_first_write
• cargo fmt --check and cargo clippy --all-targets -- -D warnings

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(requestId: serverGenReqId_7a854175-982a-4de7-b2f3-05fa3fcd62be)

[austin-barrington]

Superseded by #115, which contains this commit as part of the full Phase 1 branch (verified: this head is an ancestor of #115). Closing unmerged — merging both would apply the same change twice, since a squash-merge here creates a new commit on main while #115 still carries the original.