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.

NOTICE

@@ -10,7 +10,7 @@ This product is distributed under two licenses, applying to different artifacts.
 All source code in this repository is licensed under the Apache License,
 Version 2.0. See the LICENSE file at the repository root.
 
-This covers everything under server/, tools/, web/, and proto/.
+This covers everything under server/, web/, and proto/.
 
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 2. DICTIONARY DATA — Creative Commons Attribution-ShareAlike 4.0 International