Engine: crash reports, a log file and bug report drafts (#107)

crash: a panic hook and a locked per-run command journal (adapted from
SolidCraft's crash reports). A panic in a command becomes a report with
the model from before it and the command; one outside commands, or an
unclean exit found at the next start (abort, segfault, a crash in a
graphics driver, a kill: what #248 needs), a report with the model the
journal starts from (autosave, file or a copy) and the commands since,
the last control request and the dead run's log lines. Reports are
bounded, deduplicated by signature and replayable (crash::replay, with
writes confined to a scratch folder). The crash folder follows the
automation temp area in --control and CI runs.

logfile: a small size-rotated file logger behind the log facade.

bug_report: GitHub bug-form drafts. Version, OS and what happened by
default, home folder shown as ~; the command journal only with Share my
session, the model only with Include the model as well; a session too
long for the link becomes a file to attach and the clipboard text.

Commands: diagnostics.crash_reports, diagnostics.crash_report,
diagnostics.bug_report (Tools › Help, with its icon) and
diagnostics.crash_test (FIELDCRAFT_CRASH_TEST=1 only).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/lib.rs

@@ -13,8 +13,11 @@
 #![forbid(unsafe_code)]
 
 pub mod autosave;
+pub mod bug_report;
 pub mod catalogue;
 pub mod cmd;
+pub mod crash;
+pub mod logfile;
 
 pub use cmd::analytic_surfaces;
 pub use cmd::replace::{MESHERS, Replacement, replacement};
@@ -244,6 +247,9 @@ pub struct Session {
     pub view: fieldcraft_io::View,
     /// The view as last saved (or opened): a change to it is unsaved too.
     saved_view: fieldcraft_io::View,
+    /// This is the app's own model: its top-level commands go into the crash journal
+    /// ([`crash`]), so an unclean exit can be replayed. Off for scratch sessions and tests.
+    pub crash_journal: bool,
 }
 
 impl Session {
@@ -256,6 +262,9 @@ impl Session {
         // Session commands (open, save, undo, redo) change the session only in their last step,
         // after everything that can fail, so they need no copy of the model to restore.
         let before = (spec.kind == CommandKind::Model).then(|| (self.snapshot(), self.document()));
+        let scope = crash::Scope::enter();
+        let journaled = crash::command_begin(&scope, self, id, params);
+        let (revision, opened) = (self.revision, self.opened);
         let outcome = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| (spec.run)(self, params)));
         let result = match outcome {
             Ok(Err(EngineError::Param(p))) => Err(EngineError::Param(match spec.form {
@@ -285,6 +294,8 @@ impl Session {
                 cmd::view::forget_missing(self);
             }
         }
+        drop(scope);
+        crash::command_done(self, journaled, self.revision != revision || self.opened != opened, id, params, &result);
         result
     }