feat: make typed random-access structures opt-in (default off)

Typed structures keyed the per-encoder dictionary on per-field value WIDTH, which
caused two production failure modes: an unbounded structure explosion (OOM) on
wide/variably-typed schemas, and cross-replica/thread dictionary divergence
(decode failures: "Record id is not defined" / "Unknown constant"). Classic
shared structures are width-agnostic and bounded, and were the behavior that ran
reliably pre-4.7.13.

Disable typed-struct WRITES by default in RecordEncoder (new records use classic
shared structures); keep READS (_readStruct stays active) so existing
typed-struct data and cross-version replication still decode. Opt in per encoder
via options.randomAccessStructure. Bail the write hook via () => 0 rather than
clearing it, to preserve msgpackr's struct-safe fixint boundary.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: kriszyp

resources/RecordEncoder.ts

@@ -88,6 +88,7 @@ export class RecordEncoder extends StructonEncoder {
 	rootStore: any;
 	declare saveStructures: any;
 	declare getStructures: any;
+	declare _writeStruct: any;
 	structureUpdate?: any;
 	isRocksDB: boolean;
 	name: string;
@@ -110,6 +111,16 @@ export class RecordEncoder extends StructonEncoder {
 
 		options.structPrototype = RecordObject.prototype;
 		super(options);
+		// Typed random-access structures are opt-in as of 5.1. They keyed structures on per-field
+		// value WIDTH, so a wide/variably-typed schema minted an unbounded dictionary (OOM), and
+		// the per-encoder dictionary diverges across replicas/threads (decode failures). Disable
+		// WRITES by default — new records use classic shared structures (bounded, width-agnostic).
+		// Keep READS (_readStruct stays) so existing typed-struct data and cross-version
+		// replication still decode. Bail via () => 0 (not clearing the hook) to preserve msgpackr's
+		// struct-safe fixint boundary. Opt in per encoder with options.randomAccessStructure.
+		if (!options.randomAccessStructure && this._writeStruct) {
+			this._writeStruct = () => 0;
+		}
 		const superEncode = this.encode;
 		this.encode = function (record, options?) {
 			// this handles our custom metadata encoding, prefixing the record with metadata, including the local