ci: multi-version build matrix + multi-MC compat layer

- Parametrize build: -PmcVersion / -PapiCoordinate / -PjavaRelease / -PapiVersion;
  jar name includes mc version, plugin.yml api-version parametrized
- Compat (reflection): sendBlockChanges / getClientViewDistance /
  getStructureManager (Paper 1.19.3+); particle()/potionEffect() name lookup
  for 1.21 renames (BLOCK_CRACK->BLOCK, REDSTONE->DUST, FIREWORKS_SPARK->
  FIREWORK, JUMP->JUMP_BOOST, SLOW->SLOWNESS); drop CHERRY materials (1.20+)
- Fix Java 17 'unconditional instanceof' in JoinCommandExecutor
- capabilitiesResolution for org.bukkit:bukkit capability conflict on newer
  paper-api artifacts
- .github/workflows/build.yml: matrix for 1.19.2/1.20.1 (Java 17),
  1.21/1.21.11 (Java 21), 26.1.2/26.2 (Java 25) + quality gate
- Verified locally: default + 1.19.2/1.20.1/1.21/1.21.11 build; 26.x deps resolve

.github/workflows/build.yml

@@ -0,0 +1,65 @@
+name: Build
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  # Build a shadowJar per Minecraft version against the matching Paper API.
+  build-matrix:
+    name: Build mc-${{ matrix.mc }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          # 1.19.2 — Java 17
+          - { mc: "1.19.2",  coordinate: "io.papermc.paper:paper-api:1.19.2-R0.1-SNAPSHOT", java: 17, release: 17, api: "1.19" }
+          # 1.20.1 — Java 17
+          - { mc: "1.20.1",  coordinate: "io.papermc.paper:paper-api:1.20.1-R0.1-SNAPSHOT", java: 17, release: 17, api: "1.20" }
+          # 1.21 — Java 21
+          - { mc: "1.21",    coordinate: "io.papermc.paper:paper-api:1.21-R0.1-SNAPSHOT",   java: 21, release: 21, api: "1.20" }
+          # 1.21.11 — Java 21
+          - { mc: "1.21.11", coordinate: "io.papermc.paper:paper-api:1.21.11-R0.1-SNAPSHOT", java: 21, release: 21, api: "1.21" }
+          # 26.1.2 / 26.2 — Java 25 (new-style paper-api build artifacts; may need a
+          # Gradle wrapper >= 8.14 if the daemon rejects Java 25 — bump if so).
+          - { mc: "26.1.2",  coordinate: "io.papermc.paper:paper-api:26.1.2.build.74-stable", java: 25, release: 25, api: "1.21" }
+          - { mc: "26.2",    coordinate: "io.papermc.paper:paper-api:26.2.build.97-stable",   java: 25, release: 25, api: "1.21" }
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: ${{ matrix.java }}
+      - name: Build jar (mc ${{ matrix.mc }})
+        run: >
+          ./gradlew --no-daemon shadowJar
+          -PmcVersion=${{ matrix.mc }}
+          -PapiCoordinate='${{ matrix.coordinate }}'
+          -PjavaRelease=${{ matrix.release }}
+          -PapiVersion=${{ matrix.api }}
+      - name: Upload jar
+        uses: actions/upload-artifact@v4
+        with:
+          name: LoParkour-${{ matrix.mc }}
+          path: build/libs/LoParkour-*.jar
+
+  # Full quality gate (checkstyle + PMD + tests) on the default build.
+  quality:
+    name: Quality gate (default 1.20.4)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 21
+      - name: Full build
+        run: ./gradlew --no-daemon build
+      - name: Upload default jar
+        uses: actions/upload-artifact@v4
+        with:
+          name: LoParkour-default-1.20.4
+          path: build/libs/LoParkour-*.jar