Install the shard map on join before region movement

[austin-barrington]

Stacked on #113.

Summary

• A joining node installs the committed shard map (GET /internal/shard/map + replace_map) during join/reconnect, after metadata and before any region data movement.
• Scheduler helpers refuse movement until the joiner's map_version equals the cluster's. Catch-up does not add the joiner as a peer.

Test plan

• cargo test --lib (includes replace_map_installs_peer_snapshot, joiner_map_catchup_blocks_movement_until_versions_match)
• cargo test --test sharding_cluster_integration joiner_map_version_matches_before_region_movement
• cargo test --test sharding_cluster_integration --test sharding_integration --test sharding_failover_integration --test sharding_mv_integration
• cargo fmt --check and cargo clippy --all-targets -- -D warnings

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(requestId: serverGenReqId_ba36ae2f-978a-43a3-9ee1-637d87510e77)

[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Superseded by #115, which contains this commit as part of the full Phase 1 branch (verified: this head is an ancestor of #115).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.