Merge: a log that survives the process, so the tablet can be debugged

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

# Conflicts:
#	apps/darkroom-android/Cargo.toml
#	apps/darkroom-desktop/Cargo.toml
#	platform/dr-plat/src/lib.rs

apps/darkroom-desktop/src/main.rs

@@ -4,11 +4,21 @@
 
 use std::path::PathBuf;
 
+use dr_plat::diagnostics::Installed;
+
 fn main() -> anyhow::Result<()> {
-    env_logger::Builder::from_env(env_logger::Env::default().default_filter_or(
+    // Built rather than `init`ed, so the same logger can be handed to the
+    // diagnostics tee: `env_logger` keeps writing to stderr exactly as before,
+    // and every record it accepts is also appended to the on-disk log
+    // (NFR-OPS-1). `filter()` is asked afterwards because the environment may
+    // have overridden the default below, and the file must not be quieter than
+    // the terminal.
+    let console = env_logger::Builder::from_env(env_logger::Env::default().default_filter_or(
         "info,wgpu_core=warn,wgpu_hal=warn,zbus=warn,tracing=warn,calloop=warn,rawler=warn",
     ))
-    .init();
+    .build();
+    let level = console.filter();
+    let logging = dr_plat::diagnostics::install(Box::new(console), level);
 
     // Immediately after the logger and before anything that could fail. Until
     // now a panic on desktop went to stderr and died with the terminal, which
@@ -19,6 +29,12 @@ fn main() -> anyhow::Result<()> {
     dr_plat::crash::install(env!("CARGO_PKG_VERSION"));
 
     log::info!("DarkRoom v{}", env!("CARGO_PKG_VERSION"));
+    // First thing in the file, so a user asked for "the log" can find it
+    // without being told a path over the phone.
+    match &logging {
+        Installed::ToFile(path) => log::info!("logging to {}", path.display()),
+        Installed::ConsoleOnly(why) => log::warn!("no log file this session: {why}"),
+    }
 
     let paths: Vec<PathBuf> = std::env::args().skip(1).map(PathBuf::from).collect();
     if paths.is_empty() {