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/crash.rs

@@ -106,12 +106,7 @@ pub fn state_dir() -> PathBuf {
     if let Some(d) = STATE_DIR.get() {
         return d.clone();
     }
-    std::env::var_os("XDG_STATE_HOME")
-        .map(PathBuf::from)
-        .unwrap_or_else(|| {
-            PathBuf::from(std::env::var("HOME").unwrap_or_default()).join(".local/state")
-        })
-        .join("darkroom")
+    crate::dirs::base_dir(crate::dirs::Base::State)
 }
 
 /// Where crash records are written.