Unify PRK.Client installation data root

Use PRK_DATAROOT as the sole runtime installation-data override; unset/blank defaults to AppContext.BaseDirectory (the client executable directory, not its parent launcher directory or working directory). Resolve database, GUI, audio, effects, tweaks, layouts, textures and shaders consistently. Remove runtime AO_CLIENT_ROOT, PRK_GUIROOT and DATA_ROOT overrides; update development launcher and client docs to the unified contract. The retiring TestHarness is explicitly out of scope and unchanged. Preserve separate user preference storage and native library overrides. Acceptance: paths with spaces work, default root is independent of launch directory, and build/regression tests pass. Verified after rebasing onto develop dfb6fd700: client builds; 3,212 non-install-dependent regression tests pass (46 optional tests skipped), including path/shader, display, graphics and options tests. The options-menu merge moved preference loading into Bootstrap's constructor; its working-directory fallback is also replaced with InstallationPaths.Root. No additional installation-root overrides found in the merged runtime code. Shell syntax and diff checks pass. Full installed-game sweeps were not run. Vulkan lifetime tests now select assets through PRK_DATAROOT; their separate build was blocked by pre-existing root-owned obj files (permission denied), and display-dependent tests were not run. Pull request: #2626 — fix: Unify client installation data root and precompile Linux shaders https://git.project-rk.com/prk/engines/pulls/2626 fix/unify-client-data-root -> develop. Older shipped-asset compatibility remains deferred; no asset or music-parser changes included.