* Desktop: write log records to standard error and a rotating log file
The desktop app installed no logger, so every log::warn!/log::error! in
the workspace was discarded: the crash guard's panic report, a failed
crash-recovery save, font folders that can't be read. A launch from a
desktop menu has no terminal either, so there was nothing to attach to a
bug report.
apps/designcraft/src/logging.rs (no new dependency):
- standard error plus <settings dir>/logs/designcraft.log (beside
ui.json and prefs.json); each launch rotates the previous log to
designcraft.1.log and .2.log, so the log of a run that crashed
survives the next start; the file is capped at 16 MiB
- records logged before the settings directory is known are buffered
(bounded) and written once the file is attached
- default levels: info for designcraft*, warn for everything else;
RUST_LOG replaces them with env_logger-style directives
- never panics: an unusable directory or a failed write leaves standard
error as the only sink
The logger is installed first in main (before the engine's panic hook)
and the file is opened after argument parsing (--version leaves no
file). The settings directory moves out of prefs_path into config_dir.
A --control or DESIGNCRAFT_CONTROL_PORT value that is not a port is now
logged instead of silently leaving the control channel off. Documented
in README.md (Quick start > Logs).
* Desktop: no eprintln! on the log and panic paths
eprintln! panics when standard error is a broken pipe (a GUI launch on
Windows), and a panic inside the panic hook aborts. The "no log file"
message goes through log::warn! (standard error only by then); the panic
hook's fallback writes to standard error with write_fmt and ignores the
error. Same fix as storytold/vectorcraft#465.
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