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>

.gitignore

@@ -0,0 +1,40 @@
+### Gradle ###
+.gradle
+build/
+!gradle/wrapper/gradle-wrapper.jar
+!**/src/**/build/
+
+### IntelliJ IDEA ###
+.idea/
+*.iws
+*.iml
+*.ipr
+out/
+!**/src/**/out/
+
+.run/
+
+### Eclipse ###
+.apt_generated
+.classpath
+.eclipse/
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/**/bin/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store
+
+### Minecraft Modding ###
+run/
+!**/src/**/run/
+**/src/generated/**/.cache/
+repo/
+!**/src/**/repo/