test(good-fences): lend-the-mower journey (red)

The spec is a sequence diagram of the a2030.md story: I lend a governed
mower; the neighbor's harness shows the model only `about`, runs its
code as a tenant, and the governor stops the tipsy tenant at a 2-cell
fence. The test runs the actors through a causal tracer and compares
the trace with the diagram, then checks the yard and what the model saw.

The tracer comes from agoric-sdk PR #12895, extended to name
participants made mid-sequence (governor, tenant) and to draw thrown
errors as replies.

Red: src/ doesn't exist yet (ERR_MODULE_NOT_FOUND for src/fence.ts).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

good-fences/test/lend-the-mower.test.ts

@@ -0,0 +1,131 @@
+/** @file The lend-the-mower journey, traced and compared with its sequence diagram. */
+import './lockdown.ts';
+
+import assert from 'node:assert/strict';
+import { readFile } from 'node:fs/promises';
+import test from 'node:test';
+
+import { makeFencePage } from '../src/fence.ts';
+import { makePlayground } from '../src/governor.ts';
+import { aboutMower, makeMowerFactory } from '../src/mower.ts';
+import { makeNeighbor } from '../src/neighbor.ts';
+import { makeCompartmentLoader } from '../src/tenant-loader.ts';
+import { makeYard } from '../src/yard.ts';
+import {
+  type CausalSequenceViz,
+  makeCausalSequenceTracer,
+  md,
+  mmd,
+} from '../tools/sequence-diagram-actor-sim.ts';
+
+const designDoc = new URL('../docs/lend-the-mower.md', import.meta.url);
+
+/** a tenant that pushes until something stops it, or gives up after 20 */
+const tipsyTenant = `(env) => {
+  env.mower.start();
+  try {
+    for (let i = 0; i < 20; i += 1) env.mower.push();
+  } catch (err) {
+    return err.message;
+  }
+  return 'mowed 20 cells';
+}`;
+
+/** `{ a, b: { c } }`: keys, and the keys of object-valued members */
+const shape = (record: Readonly<Record<string, unknown>>) =>
+  `{ ${Object.entries(record)
+    .map(([key, value]) =>
+      value !== null && typeof value === 'object' && !Array.isArray(value)
+        ? `${key}: { ${Object.keys(value).join(', ')} }`
+        : key,
+    )
+    .join(', ')} }`;
+
+const quoted = (result: unknown) => `'${String(result)}'`;
+
+const viz = harden({
+  lend: {
+    label: args => {
+      const { webkey, limit } = args[0] as { webkey: string; limit: number };
+      return `lend({ webkey: '${webkey}', limit: ${limit}, say })`;
+    },
+  },
+  mower: { result: () => 'mower' },
+  govern: { args: args => `mower, ${String(args[1])}`, result: () => 'governed' },
+  chat: {
+    label: args => `chat(${shape(args[0] as Record<string, unknown>)})`,
+    result: quoted,
+  },
+  complete: {
+    label: args =>
+      `complete([${(args[0] as Record<string, unknown>[]).map(shape).join(', ')}])`,
+    result: () => 'code',
+  },
+  load: {
+    args: args => `code, { ${Object.keys(args[1] as object).join(', ')} }`,
+    result: () => 'tenant',
+  },
+  run: { result: quoted },
+}) satisfies CausalSequenceViz;
+
+test('a governed mower stops a tipsy tenant at the fence', async () => {
+  const lines = (await readFile(designDoc, 'utf8')).split('\n');
+  const section = md.skipToH(2, 'Lend a governed mower to a tipsy tenant')(lines);
+  const diagram = md.eachFence('mermaid', section).next().value;
+  if (!diagram) throw Error('lend-the-mower Mermaid block not found');
+  const documented = mmd.extractArrows(diagram);
+
+  const ES = makeCausalSequenceTracer(viz);
+  const yard = makeYard({ width: 8, height: 8 });
+  const factory = makeMowerFactory(ES, {
+    yard,
+    size: { width: 8, height: 8 },
+    mintKey: () => 'mower-key',
+  });
+  const key = factory.make({ x: 4, y: 4 });
+  const mower = factory.mower(key);
+
+  const prompts: unknown[] = [];
+  const model = harden({
+    complete(messages: readonly unknown[]) {
+      prompts.push(messages);
+      return tipsyTenant;
+    },
+  });
+  const playground = makePlayground(ES);
+  const loader = makeCompartmentLoader(ES);
+  const neighbor = makeNeighbor(ES, { model, loader });
+  const fence = makeFencePage(ES, { factory, playground, neighbor });
+
+  ES.declareParticipants({
+    F: fence,
+    MF: factory,
+    P: playground,
+    M: mower,
+    Y: yard,
+    N: neighbor,
+    L: model,
+    TL: loader,
+  });
+  ES.declareMade({ G: [playground, 'govern'], T: [loader, 'load'] });
+
+  ES.start(fence);
+  const outcome = ES(fence).lend({
+    webkey: key,
+    limit: 2,
+    say: 'you can use this mower for the weekend',
+  });
+
+  assert.deepEqual(ES.snapshot(), documented);
+  assert.equal(outcome, 'fence: 2 cells from start');
+  assert.deepEqual(yard.cutCells(), ['4,4', '4,3', '4,2']);
+  assert.deepEqual(prompts, [
+    [
+      {
+        role: 'user',
+        content: 'you can use this mower for the weekend',
+        about: { mower: aboutMower },
+      },
+    ],
+  ]);
+});