Resolve every base directory in one place, and on Windows

Five sites each read XDG_*_HOME and fell back to $HOME/.local/… on
their own, which is fine on Linux and wrong everywhere else: Windows
sets neither variable, so every one of them degraded to a path
relative to the working directory — for a Start Menu launch,
C:\Windows\System32. The models lookup walked XDG_DATA_DIRS the same
way.

dr_plat::dirs now holds the rule per platform: XDG on Unix, the known
folders on Windows — %APPDATA% for config, which roams, and
%LOCALAPPDATA% for data and state, which do not — and the executable's
own directory as the system data dir, which is where the installer
puts the models. The Android overrides stay where they were; only the
fallback behind them moved. Both rule sets are unit-tested on either
host, and the Windows one was confirmed by running the application
under Wine: its log landed in AppData\Local\darkroom\state and nothing
was written anywhere else.

platform/dr-plat/src/lib.rs

@@ -12,6 +12,7 @@
 // arrangement this crate exists to prevent.
 pub mod crash;
 pub mod diagnostics;
+pub mod dirs;
 pub mod display;
 pub mod input;
 pub mod secrets;
@@ -20,6 +21,7 @@ pub mod storage;
 pub mod volumes;
 
 pub use diagnostics::{Installed, LogFile};
+pub use dirs::{base_dir, system_data_dirs, Base};
 pub use display::{
     Bounds, DisplayInfo, DisplayProfile, DisplayServer, DisplaySurvey, FallbackReason,
     ProfileSource,