schema stamps: lazy v1 for chats + the graph family, honest refusal

Chat records gain version (CURRENT_CHAT_VERSION), stamped by a single
writeChat choke point in the store; absence reads as v1 — existing files
migrate on their next write, no mass rewrite. The graph family (four JSONL
ledgers, sources.json, render-settings.json) shares one lazy sidecar,
graph/format.json; positions stays unstamped (rebuildable) and jobs stay
unstamped (transient).

Downgrade scenario ruled honest-refusal: records stamped newer than the
running build read as absent (chats, logged) or throw (graph family) — a
read-modify-write store never rewrites what it doesn't understand.

Golden fixtures stay the unstamped legacy shape on purpose (lazy v1 is
current-valid by design); the stamped write path is pinned by
tests/schema-stamps.test.ts. Reads never write — the golden byte-identical
pin holds.

src/core/contracts/graph.ts

@@ -120,6 +120,10 @@ export interface NamingRunStats {
 	appended: number;
 }
 
+/** The graph-family format version this build writes and understands
+ *  (the format.json sidecar — src/store/graph-format.ts). */
+export const CURRENT_GRAPH_FORMAT_VERSION = 1;
+
 
 export interface GraphNode {
 	/** Opaque stable id, store-assigned: "n_" + nanoid-like. */