ci: run on the self-hosted x86_64 runner and drop the toolchain installers

Targets runs-on: flutter-android for both jobs. Since that runner is host mode
with the toolchain preinstalled, subosito/flutter-action and
actions/setup-java are removed — they would have downloaded a second Flutter
and JDK over the ones being used, and CI would no longer be building with the
toolchain the release was verified against. A Toolchain step fails loudly if
flutter or ANDROID_HOME is missing rather than letting a later step fail
obscurely.

Documents in docs/README.md why the runner cannot be the Gitea host, what host
mode trades away, and what each job protects — including why building the
release APK is necessary but not sufficient, which is the reason the JNI name
guard exists at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/ci.yaml

@@ -6,18 +6,28 @@ on:
   pull_request:
     branches: [ master ]
 
+# Both jobs target a self-hosted runner, by capability rather than by hostname.
+# See tool/setup_ci_runner.sh for how that runner is provisioned and why it is
+# not the Gitea host: Flutter cannot build an Android APK on a Linux arm64 host
+# (flutter/flutter#189724), and the Gitea box is aarch64. The runner is
+# registered at android-stuff org scope so any repo in the org can use it.
+#
+# The runner is host-mode, so Flutter, the JDK and the Android SDK/NDK are the
+# machine's own — the same toolchain that produced the verified release, and no
+# multi-hundred-MB download per run. Nothing here installs a toolchain; the
+# Toolchain step fails loudly if one is missing.
+
 jobs:
   analyze:
     name: Analyze & Test
-    runs-on: ubuntu-latest
+    runs-on: flutter-android
     steps:
       - uses: actions/checkout@v4
 
-      - uses: subosito/flutter-action@v2
-        with:
-          flutter-version: '3.47.1'
-          channel: 'stable'
-          cache: true
+      - name: Toolchain
+        run: |
+          flutter --version
+          echo "ANDROID_HOME=${ANDROID_HOME:?not set on this runner}"
 
       - name: Get dependencies
         run: flutter pub get
@@ -32,21 +42,10 @@ jobs:
 
   release-apk:
     name: Release APK
-    runs-on: ubuntu-latest
+    runs-on: flutter-android
     steps:
       - uses: actions/checkout@v4
 
-      - uses: actions/setup-java@v4
-        with:
-          distribution: 'temurin'
-          java-version: '17'
-
-      - uses: subosito/flutter-action@v2
-        with:
-          flutter-version: '3.47.1'
-          channel: 'stable'
-          cache: true
-
       - name: Get dependencies
         run: flutter pub get