Check out sibling fluxa-core repo in Platform Verification CI

Data module's Gradle build now depends on buildFluxaCoreHost (added
when UniFFI codegen moved from app to data module), which needs
../fluxa-core to exist next to the checkout. platform.yml never
checked that sibling repo out, unlike apple.yml which already does
this. Mirror apple.yml's checkout + symlink steps and workflow_dispatch
input.

.github/workflows/platform.yml

@@ -27,6 +27,12 @@ on:
       - "settings.gradle.kts"
       - "build.gradle.kts"
   workflow_dispatch:
+    inputs:
+      fluxa_core_ref:
+        description: Fluxa Rust core ref to build
+        required: false
+        default: master
+        type: string
 
 permissions:
   contents: read
@@ -39,6 +45,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - uses: actions/checkout@v4
+        with:
+          repository: FluxaMedia/fluxa-core
+          path: fluxa-core
+          ref: ${{ inputs.fluxa_core_ref || 'master' }}
+
+      - name: Link sibling Rust core
+        run: ln -sfn "$GITHUB_WORKSPACE/fluxa-core" "$(dirname "$GITHUB_WORKSPACE")/fluxa-core"
+
       - uses: actions/setup-java@v4
         with:
           distribution: temurin