CultCache: a single-file version is the hash of the record's exact stored bytes, and a copied record keeps them

A record read from a store file carries the bytes it was read from (CultPersistedRecord.RawBytes, dropped when any field is
assigned) and is written as those bytes, the way a schema catalog entry already is. A version is therefore the SHA-256 of the
slice the file holds, whatever legal encoding its writer chose, not of C#'s re-encoding, and a write that copies a record leaves
its bytes and so its version alone. The contract says exactly that, so the parity cuts port it literally.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs

@@ -306,6 +306,12 @@ public static class CultDocumentMessagePackSerialization
 
     private static void WritePersistedRecord(ref MessagePackWriter writer, CultPersistedRecord record)
     {
+        if (record.RawBytes is { } raw)
+        {
+            writer.WriteRaw(new ReadOnlySpan<byte>(raw));
+            return;
+        }
+
         writer.WriteArrayHeader(record.Variant == null ? PersistedRecordFieldCount : VariantRecordFieldCount);
         writer.Write(record.Key);
         writer.Write(record.SchemaId);
@@ -389,6 +395,7 @@ public static class CultDocumentMessagePackSerialization
 
     public static CultPersistedRecord ReadPersistedRecord(ref MessagePackReader reader)
     {
+        var start = reader.Position;
         var fieldCount = reader.ReadArrayHeader();
         var record = new CultPersistedRecord();
 
@@ -429,6 +436,7 @@ public static class CultDocumentMessagePackSerialization
             }
         }
 
+        record.RawBytes = reader.Sequence.Slice(start, reader.Position).ToArray();
         return record;
     }
 

src/GameCult.Caching/Contracts/cultcache-persistence-format.md

@@ -47,10 +47,14 @@ visible, to the committing flow or to observers, until the store has accepted
 it; a failed commit changes nothing on disk or in memory.
 
 A batch may carry conditions: per-record version, or the whole store unchanged
-since it was last loaded. A record's version is the SHA-256 of its persisted
-encoding, the array `[key, schemaId, storedAt, payload(, variant)]` a directory
-page holds; it names the schema id and storedAt as well as the bytes. storedAt is
-metadata and decides no condition. Conditions are evaluated
+since it was last loaded. A record's version is the SHA-256 of the exact bytes the
+record is stored as: in a single file the record's own slice of the file, the array
+`[key, schemaId, storedAt, payload(, variant)]` exactly as the file holds it, in
+whatever legal MessagePack encoding its writer chose (never a re-encoding of the
+decoded record); in a directory store the page the manifest indexes. A record being
+written has the version of the bytes it is written as. It names the schema id and
+storedAt as well as the payload. storedAt is metadata and decides no condition.
+Conditions are evaluated
 under the store's exclusive lock against what is durably on disk. A failed
 condition is a lost race, not an error: the commit reports it and writes
 nothing.
@@ -59,9 +63,9 @@ nothing.
 the store's exclusive lock the writer reads what is durably in the store now,
 applies only what it staged (writes and removals) and, for a commit, its batch,
 and replaces the store with the result. Every other record, and every catalog
-entry the result references, is copied forward as stored: key, schema id,
-storedAt, payload and variant, and a catalog entry's exact bytes, unknown
-fields included. No type interprets a copied record: a write may hash another
+entry the result references, is copied forward as stored: a record's exact bytes
+and a catalog entry's exact bytes, unknown fields included, so a copied record
+keeps its version. No type interprets a copied record: a write may hash another
 record's bytes to compare versions, and reads its header (key, schema id, variant
 slot), but never decodes its payload.
 A record written over one the store holds gets a storedAt later than the one it

src/GameCult.Caching/CultCache.cs

@@ -134,13 +134,24 @@ namespace GameCult.Caching
 
     public sealed class CultPersistedRecord
     {
-        public string Key { get; set; } = string.Empty;
-        public string SchemaId { get; set; } = string.Empty;
-        public string StoredAt { get; set; } = string.Empty;
-        public byte[] Payload { get; set; } = Array.Empty<byte>();
+        private string _key = string.Empty;
+        private string _schemaId = string.Empty;
+        private string _storedAt = string.Empty;
+        private byte[] _payload = Array.Empty<byte>();
+        private CultVariantDelta? _variant;
+
+        public string Key { get => _key; set { _key = value; RawBytes = null; } }
+        public string SchemaId { get => _schemaId; set { _schemaId = value; RawBytes = null; } }
+        public string StoredAt { get => _storedAt; set { _storedAt = value; RawBytes = null; } }
+        public byte[] Payload { get => _payload; set { _payload = value; RawBytes = null; } }
 
         // Non-null for a variant record (slot 4): its base and overrides. Its payload is empty.
-        public CultVariantDelta? Variant { get; set; }
+        public CultVariantDelta? Variant { get => _variant; set { _variant = value; RawBytes = null; } }
+
+        // The exact bytes this record was read from, as a store file held them. A record is written as the bytes it was read from
+        // while no field has been assigned since, so a copied record stays byte-identical and its version (the SHA-256 of its
+        // stored bytes) is the one the file's bytes name, whatever encoding their writer chose. Assigning any field drops them.
+        internal byte[]? RawBytes { get; set; }
     }
 
     // One record a write puts in a single-file store: encoded, with the catalog entry that publishes the id it is stored under and