refactor: remove unused knobs, strings and stale text

Drop what nothing uses any more: the --max-syllables flag with its
sourceSpec field, meta row and reject reason; the store's clearRejection;
the --gap CSS token; three orphaned i18n strings; two exports with no
importer; the preview npm script; the proto.yml "baseline exists" step
that has been unconditionally true since the schema landed on main; and
two root-level .gitignore entries for paths Playwright never writes.

Correct text that outlived its subject: NOTICE named a tools/ directory
that never existed, the README still pointed at the first plan and
carried a migration note for retired sources, and a few comments still
said "release" for an upstream that is a weekly export. The pin test
header now says three copies and also holds README and ATTRIBUTION to
the same URL; the Make-targets table lists clean and help.

Wire fixtures regenerated so client_hello uses the ProtocolVersion
constant and server_game_started carries the 30s turn limit.
builder_version becomes 4 for the dropped meta row.

web/tests/dictionary-source.test.js

@@ -1,10 +1,12 @@
-// The upstream dictionary URL lives in two places: the Makefile, which builds
-// it for a developer, and the Dockerfile, which builds it for the image. They
-// have to agree, or the container ships a wordlist nobody tested against.
+// The upstream dictionary URL lives in three places: the Makefile, which
+// builds it for a developer, the Dockerfile, which builds it for the image, and
+// the builder, which stamps it into the database. They have to agree, or the
+// container ships a wordlist nobody tested against. The docs that quote the
+// URL are held to the same copy.
 //
 // This lives in the JavaScript suite for no better reason than that it is the
-// suite that already reads other files in the repository. It is checking two
-// build files, not the frontend.
+// suite that already reads other files in the repository. It is checking build
+// files and docs, not the frontend.
 
 import { readFileSync } from 'node:fs';
 import { fileURLToPath } from 'node:url';
@@ -49,4 +51,11 @@ describe('the upstream dictionary export', () => {
 		const builderUrl = pin(builder, /kaikkiSourceURL\s*=\s*"([^"]+)"/, 'kaikkiSourceURL in the builder');
 		expect(builderUrl).toBe(makeUrl);
 	});
+
+	it('is the URL the docs quote', () => {
+		for (const rel of ['../../README.md', '../../data/ATTRIBUTION.md']) {
+			const doc = readFileSync(fileURLToPath(new URL(rel, import.meta.url)), 'utf8');
+			expect(doc, `${rel} does not quote DICT_URL`).toContain(makeUrl);
+		}
+	});
 });