feat(ci): two conventions become gates (improvise-w8ih, improvise-4ktt)

The stamp gate is scripts/assemble-stamp.sh, taking FILE PATTERN pairs, and
scripts/assemble-stamp-test.sh is its check_the_checker. Deleting the final
`test -z "$missing"` makes four of its eight cases fail; before the extraction
that same deletion was invisible, both misses printing while assemble.sh exited
0. It is a separate script because exercising it inside assemble.sh would need a
fabricated web/pkg, which is a second answer to "what does the site assemble
from" (improvise-vvl3). Wired into the `build` job, first, because it needs
nothing and guards a step in that job.

web/tests/bundle_freedom.test.js refuses a flat web/tests/ file that names the
bundle directory -- the constraint keeping the `js` job at seconds rather than
~4m30s, stated in prose and checked by nothing. It scans its own filename too,
so it cannot become the place a violation hides, and its companion asserts
web/tests/bundle/ still holds a test that DOES use the bundle: a scoping rule is
uninteresting if nothing falls outside it.

Both faults were injected and watched to fail; evidence in
context/test-evidence/tooling.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

web/tests/bundle_freedom.test.js

@@ -0,0 +1,45 @@
+// The `js` CI job must never need a built wasm bundle. See
+// context/code/clients.md; the split is what keeps it seconds, not ~4m30s.
+
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync, readdirSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+
+const dir = fileURLToPath(new URL(".", import.meta.url));
+
+/// Built by concatenation so this file carries no literal of the thing it
+/// forbids, and is therefore checked by itself rather than exempt from itself.
+const FORBIDDEN = "p" + "kg";
+
+/// Comments may name the directory — that is what the header above does. Only
+/// what the code reaches for is a dependency, so strip comments first, exactly
+/// as render_surface.test.js does.
+const code = (name) =>
+  readFileSync(dir + name, "utf8")
+    .replace(/\/\*[\s\S]*?\*\//g, " ")
+    .replace(/(^|[^:"'`])\/\/.*$/gm, "$1");
+
+test("no test in web/tests/ reaches for a built bundle", () => {
+  const siblings = readdirSync(dir).filter((f) => f.endsWith(".test.js"));
+  assert.ok(siblings.length > 1, `found ${siblings.length} test files — the glob moved`);
+  assert.ok(siblings.includes("bundle_freedom.test.js"), "this file must check itself");
+
+  const offenders = siblings.filter((f) => code(f).includes(FORBIDDEN));
+  assert.deepEqual(
+    offenders,
+    [],
+    `these belong in web/tests/bundle/, which the browser job runs: ${offenders}`,
+  );
+});
+
+test("the subdirectory that MAY use a bundle is not the one being checked", () => {
+  // Reading it proves the exemption is a real directory rather than a
+  // convention nobody kept, and that this check is scoped to the flat listing.
+  const nested = readdirSync(dir + "bundle").filter((f) => f.endsWith(".test.js"));
+  assert.ok(nested.length > 0, "web/tests/bundle/ is empty — the split lost its other half");
+  assert.ok(
+    nested.some((f) => readFileSync(dir + "bundle/" + f, "utf8").includes(FORBIDDEN)),
+    "no bundle test reaches for a bundle, so this check has nothing to be scoped against",
+  );
+});