bin: enable panic backtraces by default

stderr is redirected to ~/.consciousness/logs/tui-stderr.log via
redirect_stderr_to_pipe(), but the default panic hook checks
RUST_BACKTRACE before printing the trace; without the env var the
log only catches the "note: run with \`RUST_BACKTRACE=full\`" tail
and the actual frames are dropped.

Set RUST_BACKTRACE=1 programmatically before any other thread spawns
so the log captures the trace by default. Existing user-set value is
respected so callers can still opt into "full" if they want.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/bin/consciousness.rs

@@ -2,6 +2,17 @@
 #![warn(unreachable_pub)]
 
 fn main() {
+    // Force the default panic hook to print a backtrace. stderr is
+    // already redirected to a daemon log; without this the hook obeys
+    // RUST_BACKTRACE (unset by default), so the log only shows the
+    // "note: run with `RUST_BACKTRACE=full`" tail and the actual
+    // frames are lost.
+    //
+    // SAFETY: called before any other thread is spawned, so no
+    // concurrent env reader can race.
+    if std::env::var_os("RUST_BACKTRACE").is_none() {
+        unsafe { std::env::set_var("RUST_BACKTRACE", "1"); }
+    }
     std::panic::set_backtrace_style(std::panic::BacktraceStyle::Short);
     consciousness::user::main()
 }