feat: add GameContext, CastleSide, and Board.withCastle

Introduces GameContext (board + per-side CastlingRights), CastleSide enum,
and a Board.withCastle extension method. Also pins missing verification
checksums for com.fasterxml:oss-parent:41 and org.junit:junit-bom:5.9.2.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/unresolved.md

@@ -0,0 +1,23 @@
+# Unresolved Issues
+
+## [2026-03-24] JUnitSuiteLike mixin not available for ScalaTest 3.2.19 with Scala 3
+
+**Requirement / Bug:**
+CLAUDE.md prescribes that all unit tests should extend `AnyFunSuite with Matchers with JUnitSuiteLike`. However, the `JUnitSuiteLike` trait cannot be resolved in the current build configuration.
+
+**Root Cause (if known):**
+- ScalaTest 3.2.19 for Scala 3 does not provide `JUnitSuiteLike` in any public package.
+- The `co.helmethair:scalatest-junit-runner:0.1.11` dependency does not expose this trait.
+- There is no `org.scalatest:scalatest-junit_3` artifact available for version 3.2.19.
+- The trait may have been removed or changed in the ScalaTest 3.x → Scala 3 migration.
+
+**Attempted Fixes:**
+1. Tried importing from `org.scalatest.junit.JUnitSuiteLike` — not found
+2. Tried importing from `org.scalatestplus.junit.JUnitSuiteLike` — not found
+3. Tried importing from `co.helmethair.scalatest.junit.JUnitSuiteLike` — not found
+4. Attempted to add `org.scalatest:scalatest-junit_3:3.2.19` dependency — artifact does not exist in Maven Central
+
+**Suggested Next Step:**
+1. Either find the correct ScalaTest artifact/import for Scala 3 JUnit integration, or
+2. Update CLAUDE.md to reflect the actual constraint that unit tests should extend `AnyFunSuite with Matchers` (without `JUnitSuiteLike`), or
+3. Investigate whether a different test runner or configuration is needed to achieve JUnit integration with ScalaTest 3 in Scala 3