[Phase 5/6] GSI independent placement — own shards, own RF

[osvaldoandrade]

Part of epic #509. Depends on Phase 1.

Goal

A global index can declare `WITH SHARDS=N REPLICATION_FACTOR=R` so its storage / RF differs from the base. Useful when the indexed value's distribution differs sharply from the base's (e.g. base is high-cardinality user IDs, index is on a low-cardinality status enum).

Files

• `internal/placement/placement.go` — extend per-MV placement model from [Phase 5/7] MV independent placement — own shards, own RF #494 to support per-index placement.
• `internal/cluster/manager.go` — open index shards alongside base shards.
• `pkg/types/types.go` — `GlobalIndexDescriptor.Shards / ReplicationFactor`.
• `internal/sql/parser.go` — extend `CREATE INDEX AS GLOBAL ... WITH SHARDS=N REPLICATION_FACTOR=R`.

DoD

• Index with `WITH SHARDS=4 REPLICATION_FACTOR=2` opens 4 raft groups distinct from base.
• No regression for indexes that omit the placement clause.

Tier

T3 (~250 LOC). Note: probably should land after MV-5 (#494) since both share the per-object placement infrastructure.

[osvaldoandrade]

Status update post-#509 wrap

Phase 5 is deferred parallel to the MV-5 (#494) deferral. The user direction landed in #537 is GSI is RF=1 by design: pointer rows live on the index's owning leader, rebuild reconciles on owner loss. That collapses the original WITH SHARDS=N REPLICATION_FACTOR=R scope to just "per-index shard count" — useful when the indexed value's cardinality diverges from base, but no workload has surfaced the need yet.

Leaving open as a tracked-but-unprioritised idea, same posture as #494.