feat(storage): replication (#11696)

Adds experimental session sync (`@earendil-works/pi-durable/sync`).

Devices upload and pull a session's commits through a remote, reading
them from the Chord store's outbox and applying them with `applyBatch`.
If two devices continue the same session, the one that uploads second
forks to a new session.

I took out the extra sequence indexes from the earlier version of this
branch, since the Chord outbox already persists every commit, so this no
longer touches storage.

---------

Co-authored-by: Armin Ronacher <armin.ronacher@active-4.com>

packages/chord/test/sync/node-files.ts

@@ -0,0 +1,59 @@
+// `SyncFileSystem` over Node's file system, for the sync tests (a host passes its own environment, such as Pi Durable's
+// `NodeExecutionEnv`).
+
+import { appendFile, mkdir, open, readdir, readFile, rename, rm, writeFile } from "node:fs/promises";
+import { join } from "node:path";
+import type { FileResult, SyncFileInfo, SyncFileReader, SyncFileSystem } from "../../src/sync/index.ts";
+
+/** An error with a `code`: `not_found` for a missing file or directory, as `SyncFileSystem` reports it. */
+class FileError extends Error {
+	readonly code: string;
+	constructor(code: string, cause: unknown) {
+		super(cause instanceof Error ? cause.message : String(cause), { cause });
+		this.code = code;
+	}
+}
+
+async function attempt<T>(operation: () => Promise<T>): Promise<FileResult<T>> {
+	try {
+		return { ok: true, value: await operation() };
+	} catch (error) {
+		const code = (error as NodeJS.ErrnoException).code === "ENOENT" ? "not_found" : "unknown";
+		return { ok: false, error: new FileError(code, error) };
+	}
+}
+
+export const nodeFiles: SyncFileSystem = {
+	joinPath: async (parts) => ({ ok: true, value: join(...parts) }),
+	readTextFile: (path) => attempt(() => readFile(path, "utf8")),
+	readBinaryFile: (path) => attempt(async () => new Uint8Array(await readFile(path))),
+	openBinaryReader: (path) =>
+		attempt(async (): Promise<SyncFileReader> => {
+			const file = await open(path, "r");
+			return {
+				read: (offset, length) =>
+					attempt(async () => {
+						const buffer = new Uint8Array(length);
+						const { bytesRead } = await file.read(buffer, 0, length, offset);
+						return buffer.subarray(0, bytesRead);
+					}),
+				close: async () => {
+					await file.close();
+				},
+			};
+		}),
+	writeFile: (path, content) => attempt(() => writeFile(path, content)),
+	appendFile: (path, content) => attempt(() => appendFile(path, content)),
+	renameFile: (source, destination) => attempt(() => rename(source, destination)),
+	listDir: (path) =>
+		attempt(async () =>
+			(await readdir(path, { withFileTypes: true })).map(
+				(entry): SyncFileInfo => ({
+					name: entry.name,
+					kind: entry.isDirectory() ? "directory" : entry.isSymbolicLink() ? "symlink" : "file",
+				}),
+			),
+		),
+	createDir: (path, options) => attempt(async () => void (await mkdir(path, options))),
+	remove: (path, options) => attempt(() => rm(path, options)),
+};