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.

.github/workflows/proto.yml

@@ -43,23 +43,10 @@ jobs:
       - name: Lint the schema
         run: buf lint
 
-      # Skipped only until this contract first lands on main; after that the
-      # baseline always exists and the check is unconditional.
-      - name: Look for a breaking-change baseline on main
-        id: baseline
-        run: |
-          if git cat-file -e origin/main:proto/noitu/v1/game.proto 2>/dev/null; then
-            echo "exists=true" >> "$GITHUB_OUTPUT"
-          else
-            echo "exists=false" >> "$GITHUB_OUTPUT"
-            echo "::notice::no schema on main yet, skipping the breaking-change check"
-          fi
-
       # ref=origin/main, not branch=main: a pull_request checkout is a detached
       # HEAD with no local main, and branch=main fails there with a git clone
       # error rather than a breaking-change report.
       - name: Reject breaking changes
-        if: steps.baseline.outputs.exists == 'true'
         run: buf breaking --against '.git#ref=origin/main'
 
       # A committed generated tree that no longer matches the schema is worse