kv: add Reader read-only seam interface

[oc-engteam]

What

Adds kv.Reader { Get; Scan; ScanRange } — the read-only surface shared by kv.Store and kv.Snapshot — plus two compile-time assertions that both satisfy it:

var (
    _ Reader = (Store)(nil)
    _ Reader = (Snapshot)(nil)
)

Why

Store and Snapshot (added in #108) expose the identical read triple, but as distinct types a read helper written against one can't accept the other. Reader names the shared subset so a read path can be written against either the live store or a consistent point-in-time snapshot. Because Go interface assignability (not embedding) is what applies, a Store or Snapshot value is already passable where a Reader is wanted, with no explicit conversion — so this is the seam the future search-on-a-snapshot wiring will use.

Scope

Seam only. No consumer — no engine/invertedindex/documents re-parameterization, no pebblekv change. Wiring the read path onto a snapshot is a separate later change.

Compatibility

Purely additive: Store/Snapshot/Batch/Snapshotter are left byte-identical, so every existing implementer keeps satisfying them. The assertions have teeth — they'd fail to compile if Store or Snapshot ever lost a read method. No on-disk format change.

Verification (go1.24.2)

• core build green — the var _ Reader = … assertions are the compile-time check.
• gofmt clean; diff is a pure 18-line addition to core/kv/kv.go, zero deletions.
• No behavioral test (interface-only).

Built via the SDD flow (spec → review → workflow-driven implementation).

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