Promote Queue to a first-class model (backed by `ImmutableEdge`)

[em3s]

Context

A queue isn't its own model. It's an ImmutableEdge table with a queue/v1 marker + numPartitions in the table comment, decoded at runtime (QueueMetadataService, QueueService.getNumPartitions). The hierarchy — Edge / ImmutableEdge / MultiEdge / Vertex — has no Queue.

So queue capability is invisible on the descriptor. On the unified collection surface (#470), clients can't tell which sub-resources a collection exposes.

Decision

Make Queue a first-class model (a collection kind). Add it to the type hierarchy; the descriptor declares kind=Queue. The queue shape (partition/seq/value, numPartitions) becomes the model definition.

• Implementation and storage reuse ImmutableEdge as-is — enqueue/poll already run over it, so no engine change. The comment marker stays as storage encoding; only the model layer becomes explicit.
• Data face is messages only; /edges stays hidden as an implementation detail.

Consequences

• kind=Queue on the descriptor tells clients /messages, partitions, and poll exist — closes the Settle v3 vocabulary on database/collection (table reserved for HBase) #470 discoverability gap.
• Queue gets its own schema, validation, and lifecycle, separate from ImmutableEdge.
• Engine and storage untouched; only the type system, descriptor, and server surface change.
• Open: Queue↔ImmutableEdge code relationship (delegation vs subtype), whether to fully hide /edges.