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.

testdata/fixture-words.txt

@@ -2,7 +2,7 @@
 #
 # Hand-written common Vietnamese compounds, not an extract of the upstream
 # dictionary: CI must never download the upstream wordlist, and a fixture derived
-# from it would carry that release's licence into every test artifact.
+# from it would carry that export's licence into every test artifact.
 #
 # `make fixture-dict` turns this into a real derived database through the same
 # filter, alias and write path production uses, so a fixture cannot drift into