Document the read-only engine user-folder caveat

The non-editable install requires SNAP/SNAP_BUILD for engine-path resolution
(now in the wrapper). The engine-level user folder (cache/metrics/UserSettings)
still resolves under read-only /app for the projectless Project Manager; tested
that the SourceProjectUserPath regset is overwritten by O3DE's bootstrap and
does not help. These writes are non-fatal - documenting rather than chasing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -147,11 +147,24 @@ These were confirmed by inspecting the v26.05 package (`opt/O3DE/26.05/…`):
   project-build CMake-configure path.) If a future release renames the package or
   bumps its version away from `1.0.0`, the build fails fast — those are the lines
   to re-check.
-- **Other runtime writes into the install tree may still fail.** Anything else
-  O3DE tries to generate *inside* `/opt/O3DE/...` at runtime (e.g. certain per-gem
-  Python deps, or the engine-level asset cache) hits the read-only `/app`. Project
-  data lives in your writable home dir, so normal project work should be fine;
-  this is the main remaining open risk.
+- **Engine-path resolution needs `SNAP`/`SNAP_BUILD`.** Because the `o3de` CLI is
+  installed non-editably, its package lives in the per-user venv, so O3DE's
+  `manifest.get_this_engine_path()` (which walks up from the package `__file__`)
+  would resolve into the venv and fail to find `engine.json`. O3DE has a built-in
+  override for immutable installs: when `SNAP` and `SNAP_BUILD` are set it returns
+  `$SNAP/$SNAP_BUILD`. The wrapper sets them to the engine root. (Confirmed these
+  vars are read *only* there — no other snap-specific behaviour is triggered.)
+- **The Project Manager's own user folder is read-only (cosmetic).** With no
+  project open, O3DE puts its engine-level *user* folder (asset cache, metrics,
+  `UserSettings.xml`) under `<engine>/.../user`, i.e. read-only `/app`. You'll see
+  `Couldn't find a valid asset cache folder…`, `cannot write to temp file …
+  UserSettings.xml`, and a metrics warning at startup. These are **non-fatal** —
+  the Project Manager runs fine; it just doesn't persist its *own* window/UI
+  settings between runs. The documented override
+  (`/O3DE/Runtime/FilePaths/SourceProjectUserPath`) does **not** help: O3DE
+  recomputes that path from the engine root during bootstrap and overwrites a
+  `--regset`. Real project data (projects you create, their caches) lives under
+  your home directory and is fully writable, so actual work is unaffected.
 - **Launcher icon.** Shipped as `org.o3de.O3DE.png` (the `.deb` itself has only
   in-editor asset icons). It's kept *inside* the app so the running window/taskbar
   shows it, but removed from the *exported* set because `flatpak build-export`