Function getAppMountPath

The absolute path where this app's own repository filesystem is mounted (FILE_SHARING_SPEC §11.2). Prefer this over hardcoding /app: the repo is dual-mounted at both /app (back-compat) and its canonical /mnt/{hash} address, and this returns the canonical one the host reports. Falls back to /app when the host hasn't reported a canonical path (older host / before the report arrives) — both paths are live, so either resolves the same files.