// Package projection is the domain-neutral substrate for the "snapshot stream +
// local index" seam: one authoritative producer publishes a framed
// begin → chunk* → end snapshot (plus per-batch deltas) on the bus, and N
// receivers each assemble their OWN cold-start-ready, mmap-backed .bin index
// from it. No shared volume; each consumer is independently eventually
// consistent.
//
// It was extracted from platform/catalog (which now re-exports these types for
// its Projection wire shape) so other domains can ride the same rails without
// depending on catalog. The pieces:
//
//   - SnapshotWriter[T] / SnapshotReader[T] (snapshot.go) — the value-agnostic
//     key→blob .bin format: sorted records, binary-search Get, full-walk Scan,
//     mmap on Unix. Generic over the stored type T.
//   - SnapshotHolder[T] (manager.go) — race-free atomic hot-reload of a reader
//     under concurrent Get/Scan, with a drain-grace close-batching reaper.
//   - Store[W, T] (store.go) — the consumer state machine: snapshot assembly,
//     epoch gating, same-epoch delta overlay (with delete tombstones), and the
//     completeness guard. Generic over the WIRE type W (decoded by a
//     domain-supplied Decoder) and the STORED type T (projected by a Transform).
//   - The bus framing contract (framing.go) — Meta* keys, snapshot phases, and
//     the decoded Update[W] shape.
//
// The framing is fixed and shared; the on-the-wire payload schema is the
// domain's own concern, bridged by Decoder[W]. See
// docs/plan-projectionstore-generalization.md for why this boundary exists.
package projection