Step 2: raise a second scenario, and share the invariants

The suite next door raises one scenario and asks deep questions of it. This one
asks shallow questions of every scenario — the half that was missing, since both
faults found by hand lived in scenarios nothing ever built.

Adds bootstrap-single, the cheapest, and the loop that lets the list grow. Also
adds the second universal invariant: every address a scenario declared is one
the machine actually holds. A machine that came up bare looks identical to one
that came up correctly until something asks it.

Verified to bite rather than assumed: against a live instance, the real
declaration passes and a declaration claiming an address nothing holds fails
with 'anchor declared 192.0.2.99 on hosting but holds 192.0.2.10'.

Integration now runs with --test-concurrency=1. Two files raise real instances,
node --test runs files in parallel by default, and two concurrent runs of this
suite already produced a whole-suite failure once — every test red, from
resource contention rather than from any fault in the code.

Gate: 45.7s -> 60.2s.

package.json

@@ -9,7 +9,7 @@
   "scripts": {
     "typecheck": "tsc --noEmit && tsc --noEmit -p tsconfig.test.json",
     "test": "node --test --experimental-strip-types 'test/*.test.ts'",
-    "test:integration": "node --test --experimental-strip-types 'test/integration/*.test.ts'",
+    "test:integration": "node --test --test-concurrency=1 --experimental-strip-types 'test/integration/*.test.ts'",
     "check": "npm run typecheck && npm test && npm run test:integration"
   },
   "dependencies": {