[BFTree] Add RangeIndex cluster migration

[tiagonapoli]

RangeIndex keys are backed by a live BfTree instance whose on-disk state lives in a separate file (data.bftree) and whose main-store record is a 35-byte stub carrying a process-local TreeHandle. Shipping just the stub bytes is not enough — the destination needs both the tree's on-disk snapshot and a freshly-recovered local TreeHandle.

This change adds end-to-end migration for MIGRATE SLOTS, mirroring the Vector Set migration pattern but transferring a GZip-compressed native snapshot of the tree in chunks over the existing migration transport.

Wire format (piggybacks on TryWriteRecordSpan):

• New MigrationRecordSpanType.RangeIndexSnapshotChunk (4): [int keyLen][key][int chunkIndex][int totalChunks][long uncompressedLen][int chunkLen][bytes]
• New MigrationRecordSpanType.RangeIndexStub (5) — finalizer: [int keyLen][key][35-byte stub]

Source side:

• MigrateScanFunctions.Reader captures RangeIndex records (RecordType == 2) onto MigrateOperation.RangeIndexes instead of the regular DiskLogRecord path.
• RangeIndexManager.SnapshotForMigration acquires the per-tree exclusive lock, native-snapshots the tree to migrate.bftree, releases the lock, then GZip-compresses to migrate.bftree.gz.
• MigrateSessionRangeIndex.TransmitRangeIndex streams 256 KB chunks followed by the stub finalizer, deletes the source main-store key, and cleans up temporary files.
• MigrateSessionSlots loops over migrateOperation.SelectMany(mo => mo.RangeIndexes) after the Vector Set block.

Destination side:

• RangeIndexManager.HandleMigratedRangeIndexChunk appends chunks to restore.bftree.gz (validates monotonic ordering per key).
• RangeIndexManager.HandleMigratedRangeIndexStub decompresses to data.bftree, recovers a fresh BfTreeService via BfTreeService.RecoverFromSnapshot, rewrites the stub's TreeHandle to the new local pointer, issues an RICREATE RMW inline (using StorageSession.CompletePendingForSession), and registers the tree on success.
• RespClusterMigrateCommands.NetworkClusterMigrate dispatches both new span types; the stub branch also validates the key's hash slot is importing.

Not included in this PR (tracked separately):

• MIGRATE KEYS path — explicit per-key command requires a pre-probe to detect RangeIndex keys before TransmitKeys. To be added once the SLOTS path is validated end-to-end.
• AOF replication of migrated trees to destination replicas.
• Cluster migration integration tests.