Introduce atomic-snapshot persistence types that will replace the current
fine-grained 13-proc Persistence interface. This commit is purely additive:
no existing call site changes, no behaviour change.
New types (sds/types/):
- channel_meta.nim — ChannelMeta (atomic per-channel snapshot blob),
ChannelData (bootstrap payload), OutgoingRepairKV / IncomingRepairKV
(flattened map entries for protobuf wire shape).
- history_update.nim — HistoryUpdate (combined append/evict payload for
the message log).
New codec (sds/snapshot_codec.nim):
- Protobuf encode/decode for all new types, reusing the existing
SdsMessage and HistoryEntry encoders from sds/protobuf.nim.
- Explicit schemaVersion=1 on ChannelMeta; decoder rejects unknown
versions loudly rather than silently truncating.
- Time encoded as int64 unix milliseconds.
Tests (tests/test_snapshot_codec.nim):
- 13 round-trip cases covering empty, single-entry, full-buffer, and
repair-heavy snapshots; ChannelData ordering; HistoryUpdate variants;
schemaVersion rejection.
Planning artefacts:
- ANALYSIS_SDS_PERSISTENCE.md — problem statement (partial-write
divergence, chatty call rate, non-fatal-error policy gap).
- ANALYSIS_SNAPSHOT_SAVE_POINTS.md — exact save points per protocol op
and projected call rates.
- PLAN_SNAPSHOT_PERSISTENCE.md — phased refactor plan; this commit
implements phase 0.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
