M0: scaffold bnstoolkit mod (NeoForge 1.21.1)

Empty mod that loads cleanly on client + dedicated server. Sets up the
package layout and milestone TODOs for the BNS in-game UI work.

- uk.sijbers.bnstoolkit.{BnsToolkit, BnsToolkitClient} mod entries
- 5 placeholder screens extending a shared BaseBnsScreen
  (TierUpgrade, BountyBoard, QuestLog, PlotPurchase, ShopBrowser)
- 3 key bindings (T/B/P) registered, handlers TODO M1+
- SpurHud overlay placeholder for M4
- Network: payload registrar + one smoke-test C2S/S2C pair
  (RequestTierState / TierStateUpdate)
- en_us.json for screen + keybind labels
- neoforge.mods.toml pinned to neo_version 21.1.228 (live server match)

Build verified: ./gradlew build -> bnstoolkit-0.1.0.jar (15.6 KB).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -0,0 +1,25 @@
+name: Build
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          fetch-tags: true
+
+      - name: Setup JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: '21'
+          distribution: 'temurin'
+
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Build with Gradle
+        run: ./gradlew build