Add documentation for rules_apple workspace separation (#5905)

Documents why SparklePlugin is built in a separate workspace due to
rules_swift version conflicts, and provides a checklist for when/how
to reintegrate it into the main workspace.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

docs/RULES_APPLE_WORKSPACE_SEPARATION.md

@@ -0,0 +1,150 @@
+# rules_apple Workspace Separation
+
+This document explains why `rules_apple` is built in a separate Bazel workspace (`sparkle_workspace/`) and what conditions need to be met before it can be reintegrated into the main workspace.
+
+## Background
+
+The main workspace cannot include `rules_apple` due to transitive dependency conflicts with `rules_swift`. Multiple dependencies require different major versions of `rules_swift`:
+
+| Dependency | rules_swift Version | Compatibility Level |
+|------------|---------------------|---------------------|
+| grpc | 3.x | 3 |
+| flatbuffers | 2.x | 2 |
+| rules_apple | 2.x | 2 |
+
+Bzlmod's `single_version_override` can force a single version, but compatibility levels 2 and 3 are incompatible. Forcing `rules_swift` 3.x (required for grpc) breaks `rules_apple` and `flatbuffers`.
+
+## Current Solution
+
+The `SparklePlugin` (the only component requiring `rules_apple`) is built in an isolated workspace:
+
+```
+sparkle_workspace/
+├── MODULE.bazel      # Minimal deps: apple_support, rules_apple, sparkle
+├── BUILD.bazel       # Builds SparklePlugin.bundle
+├── SparklePlugin.m   # Objective-C source
+├── Info.plist        # Bundle metadata
+└── external/
+    └── BUILD.sparkle # Build file for Sparkle framework
+```
+
+### How It Works
+
+1. **Build script**: `scripts/build_sparkle_plugin.sh` builds the plugin from the separate workspace
+2. **Output**: The built `SparklePlugin.bundle` is placed in `Assets/Plugins/macOS/`
+3. **CI integration**: `mac_build.yml` calls `inject_sparkle.sh` to embed Sparkle into the Mac app
+
+The main workspace uses `single_version_override` for `rules_swift` 3.x to satisfy grpc, and includes a comment noting that `rules_apple` is intentionally excluded.
+
+## What SparklePlugin Does
+
+SparklePlugin is a native macOS library that:
+- Initializes the [Sparkle](https://sparkle-project.org/) auto-update framework
+- Exposes C functions for Unity to call via P/Invoke:
+  - `SparklePlugin_Initialize`
+  - `SparklePlugin_CheckForUpdates`
+  - `SparklePlugin_CheckForUpdatesInBackground`
+  - `SparklePlugin_IsCheckingForUpdates`
+  - `SparklePlugin_GetAutomaticallyChecksForUpdates`
+  - `SparklePlugin_SetAutomaticallyChecksForUpdates`
+  - `SparklePlugin_IsRunningFromReadOnlyVolume`
+  - `SparklePlugin_ShowDMGWarning`
+
+## Conditions for Reintegration
+
+To move `rules_apple` back into the main workspace, **all** of the following must be true:
+
+### 1. rules_swift Version Alignment
+
+Check if grpc, flatbuffers, and rules_apple all support the same `rules_swift` major version:
+
+```bash
+# Check what rules_swift version each dependency requires
+bazel mod graph 2>&1 | grep -A2 "rules_swift"
+```
+
+Or check BCR directly:
+- https://registry.bazel.build/modules/grpc
+- https://registry.bazel.build/modules/flatbuffers
+- https://registry.bazel.build/modules/rules_apple
+
+**Test**: After updating versions, verify you can add this to MODULE.bazel without errors:
+```python
+bazel_dep(name = "rules_apple", version = "X.Y.Z")
+```
+
+### 2. Build Test
+
+If the dependency can be added, test that both the main build and SparklePlugin work:
+
+```bash
+# Main workspace builds
+bazel build //src/main/scala/net/eagle0/eagle:eagle_server
+bazel build //src/main/cpp/net/eagle0/shardok:shardok-server
+bazel build //ci:eagle_server_image
+
+# SparklePlugin builds (would move to main workspace)
+bazel build //sparkle:SparklePlugin  # After moving files
+```
+
+### 3. Files to Move
+
+When reintegrating, move these from `sparkle_workspace/` to the main workspace:
+
+| Source | Destination |
+|--------|-------------|
+| `sparkle_workspace/SparklePlugin.m` | `src/main/objc/net/eagle0/sparkle/SparklePlugin.m` |
+| `sparkle_workspace/Info.plist` | `src/main/objc/net/eagle0/sparkle/Info.plist` |
+| `sparkle_workspace/BUILD.bazel` | `src/main/objc/net/eagle0/sparkle/BUILD.bazel` |
+| `sparkle_workspace/external/BUILD.sparkle` | (inline into MODULE.bazel http_archive) |
+
+### 4. MODULE.bazel Changes
+
+Add to the main MODULE.bazel:
+
+```python
+bazel_dep(name = "rules_apple", version = "X.Y.Z", repo_name = "build_bazel_rules_apple")
+
+# Sparkle framework
+http_archive(
+    name = "sparkle",
+    build_file_content = """...""",  # Contents from external/BUILD.sparkle
+    sha256 = "...",
+    url = "https://github.com/sparkle-project/Sparkle/releases/download/...",
+)
+```
+
+Remove the `single_version_override` for `rules_swift` if no longer needed.
+
+### 5. Update Build Scripts
+
+- Update `scripts/build_sparkle_plugin.sh` to build from main workspace
+- Update `scripts/build_plugins.sh` if it references the separate workspace
+- Verify `mac_build.yml` still works
+
+### 6. Cleanup
+
+After successful integration:
+```bash
+rm -rf sparkle_workspace/
+```
+
+Update comments in MODULE.bazel that reference the separate workspace.
+
+## Version History
+
+| Date | Event |
+|------|-------|
+| 2026-02-04 | Separated sparkle_workspace (PR #5882) to enable Bazel 8 upgrade |
+| 2026-02-05 | Upgraded to Bazel 8.5.1 (PR #5883) |
+
+## Monitoring
+
+Periodically check if the upstream dependencies have aligned:
+
+1. **grpc releases**: https://github.com/grpc/grpc/releases
+2. **flatbuffers releases**: https://github.com/google/flatbuffers/releases
+3. **rules_apple releases**: https://github.com/bazelbuild/rules_apple/releases
+4. **BCR updates**: https://registry.bazel.build/
+
+When a new version of any of these is released, check if the `rules_swift` requirements have converged.