Add optional export filename, tests for post-implement features

Add optional filename field to export dialog with automatic .json
extension handling. Extract resolveFilename() for testability. Add
tests for includeHistory flag, bundleToJson, and filename resolution.
Add export format compatibility note to CLAUDE.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/__tests__/export-import.test.ts

@@ -9,10 +9,13 @@ import {
 import { describe, expect, it } from "vitest";
 import {
 	assembleExportBundle,
+	bundleToJson,
+	resolveFilename,
 	validateImportBundle,
 } from "../persistence/export-import.js";
 
 const ISO_TIMESTAMP_RE = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}/;
+const DEFAULT_FILENAME_RE = /^initiative-export-\d{4}-\d{2}-\d{2}\.json$/;
 
 const encounter: Encounter = {
 	combatants: [
@@ -110,6 +113,82 @@ describe("assembleExportBundle", () => {
 	});
 });
 
+describe("assembleExportBundle with includeHistory", () => {
+	it("excludes undo/redo stacks when includeHistory is false", () => {
+		const bundle = assembleExportBundle(
+			encounter,
+			undoRedoState,
+			playerCharacters,
+			false,
+		);
+		expect(bundle.undoStack).toHaveLength(0);
+		expect(bundle.redoStack).toHaveLength(0);
+	});
+
+	it("includes undo/redo stacks when includeHistory is true", () => {
+		const bundle = assembleExportBundle(
+			encounter,
+			undoRedoState,
+			playerCharacters,
+			true,
+		);
+		expect(bundle.undoStack).toEqual(undoRedoState.undoStack);
+		expect(bundle.redoStack).toEqual(undoRedoState.redoStack);
+	});
+
+	it("includes undo/redo stacks by default", () => {
+		const bundle = assembleExportBundle(
+			encounter,
+			undoRedoState,
+			playerCharacters,
+		);
+		expect(bundle.undoStack).toEqual(undoRedoState.undoStack);
+	});
+});
+
+describe("bundleToJson", () => {
+	it("produces valid JSON that round-trips through validateImportBundle", () => {
+		const bundle = assembleExportBundle(
+			encounter,
+			undoRedoState,
+			playerCharacters,
+		);
+		const json = bundleToJson(bundle);
+		const parsed: unknown = JSON.parse(json);
+		const result = validateImportBundle(parsed);
+		expect(typeof result).toBe("object");
+	});
+});
+
+describe("resolveFilename", () => {
+	it("uses date-based default when no name provided", () => {
+		const result = resolveFilename();
+		expect(result).toMatch(DEFAULT_FILENAME_RE);
+	});
+
+	it("uses date-based default for empty string", () => {
+		const result = resolveFilename("");
+		expect(result).toMatch(DEFAULT_FILENAME_RE);
+	});
+
+	it("uses date-based default for whitespace-only string", () => {
+		const result = resolveFilename("   ");
+		expect(result).toMatch(DEFAULT_FILENAME_RE);
+	});
+
+	it("appends .json to a custom name", () => {
+		expect(resolveFilename("my-encounter")).toBe("my-encounter.json");
+	});
+
+	it("does not double-append .json", () => {
+		expect(resolveFilename("my-encounter.json")).toBe("my-encounter.json");
+	});
+
+	it("trims whitespace from custom name", () => {
+		expect(resolveFilename("  my-encounter  ")).toBe("my-encounter.json");
+	});
+});
+
 describe("round-trip: export then import", () => {
 	it("produces identical state after round-trip", () => {
 		const bundle = assembleExportBundle(