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/lockdown.ts

@@ -0,0 +1,4 @@
+/** @file Harden the realm before any test module runs: tenants share it. */
+import 'ses';
+
+lockdown();