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>

.gitattributes

@@ -0,0 +1,5 @@
+# Disable autocrlf on generated files, they always generate with LF
+# Add any extra files or paths here to make git stop saying they
+# are changed when only line endings change.
+src/generated/**/.cache/* text eol=lf
+src/generated/**/*.json text eol=lf