Mark WilkensXL build versions

docs/wilkensxl-build.md

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+# WilkensXL build versioning
+
+Odysseus keeps the upstream application version separate from the local
+WilkensXL build version.
+
+- `APP_VERSION` and `UPSTREAM_VERSION` track the upstream release.
+- `BUILD_VERSION` and `FULL_VERSION` identify this custom image.
+- `/api/version` returns both values so Dockge, logs, and the UI can show which
+  build is running.
+
+The default custom build is:
+
+```text
+1.0.1-wilkensxl.2
+```
+
+The value can be overridden at runtime with:
+
+- `ODYSSEUS_BUILD_FLAVOR`
+- `ODYSSEUS_BUILD_REVISION`
+- `ODYSSEUS_BUILD_VERSION`
+
+## Update workflow
+
+When upstream publishes a new Odysseus version:
+
+1. Merge or rebase the upstream changes into the WilkensXL branch.
+2. Keep the addon loader small and stable.
+3. Put local behavior changes into addons wherever possible.
+4. Run the addon, integration, readiness, and version metadata tests.
+5. Bump `BUILD_REVISION` or set `ODYSSEUS_BUILD_VERSION` for the new image.
+
+The loader does not remove all porting work, but it reduces it. Most local
+behavior should live behind addon manifests, so future upstream updates only
+need the loader compatibility checked plus any addons that touch changed
+internals.