Generate and verify Standalone (Linux, Mac, Windows) and Mobile (iOS, Android) builds

**What**

Manually generate player builds for all target platforms — Standalone (Linux, Mac, Windows) and Mobile (iOS, Android) — using the Open Creator Rails sample scene, and verify each build runs without errors. No CI integration; this is a hands-on local build verification task.

**Why**

Before v1.0 release, we need confidence that the SDK and sample compile and run correctly across all shipping platforms. Build errors, missing platform-specific stubs, or runtime crashes must be caught before the package is published.

**How**

1. Open the project in Unity 6000.3 with the Build Profiles window (`File > Build Profiles`)
2. For each platform below, switch the active platform, configure the settings listed, trigger a build, and record the outcome:

**Standalone — Windows (x86_64)**
- Scripting Backend: IL2CPP
- Architecture: x86_64
- Build and run; confirm the sample scene loads and SDK initialises without console errors

**Standalone — macOS (Apple Silicon + Intel)**
- Scripting Backend: IL2CPP
- Architecture: Universal (arm64 + x86_64)
- Build and run; confirm the sample scene loads and SDK initialises without console errors

**Standalone — Linux (x86_64)**
- Scripting Backend: IL2CPP
- Architecture: x86_64
- Build and run; confirm the sample scene loads and SDK initialises without console errors

**Mobile — iOS**
- Scripting Backend: IL2CPP
- Architecture: ARM64
- Generate Xcode project; open in Xcode, build to device or simulator, confirm the sample scene loads and SDK initialises without console errors

**Mobile — Android**
- Scripting Backend: IL2CPP
- Architecture: ARM64
- Target API: minimum 24, target 35
- Build APK; install on device or emulator, confirm the sample scene loads and SDK initialises without console errors

3. Log any build errors, linker warnings, or runtime crashes as child issues and link them here before marking this task complete

**Acceptance Criteria**

- All five platforms produce a successful build with no compilation errors
- The sample scene runs on each platform without SDK-related runtime errors or exceptions
- Any platform-specific issues found are logged as linked issues

**Estimation**

**Dependencies**

- [#14] v1.0 Release — builds should target the v1.0 release candidate state of the package