db/kv, db/state: add an additive RangeReader interface and fold in the harness range machinery

[mh0lt]

Motivation

#22467 introduced membatchwithdb.DomainReader — an overlay→SharedDomains bridge for point domain reads (GetAsOf, HistorySeek), wired via overlay.DomainReader = sd. It's the clean mechanism for making overlay read views observe in-flight (not-yet-committed) tip state.

#21414 converged on DomainReader for point reads, but it still carries a separate in-memory range/history/index machinery for the RPC test harness, because DomainReader does not cover those methods:

• SharedDomains.RangeAsOf / HistoryRange / IndexRange (+ TemporalMemBatch helpers: memRangeAsOf, memHistoryRange, memIndexTxNums, iiMem, liveLimitKV, …)
• the harness OverlayDB / sdRoTx wrapper that routes RPC reads through those.

So today there are two overlapping overlay-read mechanisms: DomainReader (point reads) and the harness range machinery (range/history/index).

Proposal — additive RangeReader, not a fattened DomainReader

Keep DomainReader focused on point reads and add a separate, additive interface for the range/history/index surface (interface segregation — a consumer implements only what it serves):

type RangeReader interface {
    RangeAsOf(ctx, domain, fromKey, toKey, ts, asc, limit, roTx) (stream.KV, error)
    HistoryRange(ctx, domain, fromTs, toTs, asc, limit, roTx) (stream.KV, error)
    IndexRange(name, k, fromTs, toTs, asc, limit, roTx) (stream.U64, error)
}

• MemoryMutation gains an optional RangeReader field alongside DomainReader; overlay.RangeReader = sd wires the SD (which already implements these).
• The overlay/read-view range methods route through RangeReader when set, exactly as point reads route through DomainReader.
• (Latest reads GetLatest/HasPrefix can join DomainReader or a small LatestReader — decide when wiring.)

Then the harness OverlayDB/sdRoTx and the standalone SharedDomains.RangeAsOf/HistoryRange/IndexRange delegations fold into the RangeReader path, removing the duplicate mechanism. DomainReader stays small; nothing that only needs point reads has to grow.

Scope / sequencing

Do this alongside (or after) the bg-commit-default flip, where in-flight range reads actually matter. Until then #21414 keeps the range machinery as-is. Relates to #21414 (converged on DomainReader for point reads), #22467 (introduced DomainReader), #22520 (SD run-task worker), #21314 (SD-aware consumers).

[VBulikov]

yperbasis added Imp2 as Importance

[VBulikov]

VBulikov added Imp2 as Importance