feat(server): counters, readiness, drain mode and a version stamp

expvar counters for connections, rooms, games, submissions by rejection
reason, eliminations, chat, joins and bot moves, served on a separate
debug address so they never sit on the public mux, plus one structured
word_rejected log line per refused word carrying the normalized word and
its link. GET /readyz flips to 503 while draining; SIGTERM stops new
rooms, waits up to NOITU_DRAIN_TIMEOUT for live games, then shuts down.
GET /version and the startup log carry the build's git describe.

Fuzz targets for the frame decoder, the text sanitizer and Vietnamese
normalization; the last one found that composing before lowercasing
could leave a non-NFC result, now recomposed after lowering.

CI runs on dev as well as main, gates gofmt and golangci-lint, tracks the
buf major instead of an exact pin, and dependabot watches every
ecosystem. The lint findings that had been hidden by the default
per-issue cap are fixed.

.github/workflows/proto.yml

@@ -7,7 +7,7 @@ name: proto
 
 on:
   push:
-    branches: [main]
+    branches: [main, dev]
   pull_request:
 
 permissions:
@@ -22,9 +22,12 @@ jobs:
           # buf breaking compares against main, which needs real history.
           fetch-depth: 0
 
+      # A moving version rather than an exact pin, per house rule: updates
+      # arrive on the next run, and a breaking major would surface here before
+      # it could surprise a contributor's own machine.
       - uses: bufbuild/buf-setup-action@v1
         with:
-          version: 1.69.0
+          version: latest
 
       - uses: actions/setup-go@v5
         with: