docs/design: promote snapshot_logical_decoder proposed→partial

Phase 0a (decoder) has fully shipped (internal/backup/ +
cmd/elastickv-snapshot-decode, PRs #790/#791/#792/#806/#810). Per the
docs/design/README lifecycle convention, promote the status token in a
dedicated commit and add a lifecycle note pointing at the Phase 0b
encoder design doc. Format ownership stays with this doc.

Author: bootjp

…_29_proposed_snapshot_logical_decoder.md …4_29_partial_snapshot_logical_decoder.mddocs/design/2026_04_29_proposed_snapshot_logical_decoder.md renamed to docs/design/2026_04_29_partial_snapshot_logical_decoder.md docs/design/2026_04_29_proposed_snapshot_logical_decoder.md renamed to docs/design/2026_04_29_partial_snapshot_logical_decoder.md

@@ -1,9 +1,17 @@
 # Snapshot ↔ Logical-Format Decoder (Phase 0)
 
-Status: Proposed
+Status: Partial
 Author: bootjp
 Date: 2026-04-29
 
+> **Lifecycle (2026-05-25):** Phase 0a (decoder) has fully shipped —
+> `internal/backup/` + `cmd/elastickv-snapshot-decode` (PRs #790,
+> #791, #792, #806, #810). Phase 0b (encoder) is specified in detail
+> in `2026_05_25_proposed_snapshot_logical_encoder.md` and is not yet
+> implemented. Phase 0c (operator integration) is open. This doc
+> remains the format owner; the encoder doc owns the reverse-direction
+> wire-format reconstruction.
+
 ## Background
 
 The existing FSM snapshot path (`store/snapshot_pebble.go`, see