Robustness: VectorCraft never crashes — no panics in shipped code

Shipped code no longer unwraps, expects or panics: every one of the 124
unwrap()/expect()/panic!/unreachable! sites outside tests now handles the
failure (Result/Option, ?, let-else, defaults) and reports errors to the user.

- Lints: workspace clippy denies unwrap_used, expect_used, panic, todo,
  unimplemented and unreachable; clippy.toml allows them in tests, and
  integration tests, examples and testkit opt out explicitly.
- Safety net (vectorcraft_engine::guard): a panic in a top-level command rolls
  the document back and returns EngineError::Internal; a panicking tool is
  reset; the MCP server answers with an internal error and keeps serving; the
  UI frame and each control request are guarded.
- APIs that hid failures now return them: Rendered::to_png/to_jpeg/to_webp
  return Result (JPEG/WebP errors used to write empty files),
  FontDb::face returns Option, Region::top returns Option.
- Fuzzing: engine/tests/import_fuzz.rs feeds garbage, hostile and mutated SVG
  and PDF through import → render → export (30k cases locally, no panics).
- Docs: AGENTS.md/CLAUDE.md rule, docs/development.md "Robustness" section,
  README pointer and a PR template checklist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -120,12 +120,12 @@ pub fn has_doc(s: &Session) -> std::result::Result<(), String> {
     s.active().map(|_| ()).ok_or_else(|| "no document open".into())
 }
 pub fn has_selection(s: &Session) -> std::result::Result<(), String> {
-    has_doc(s)?;
-    if s.active().unwrap().selection.is_empty() { Err("nothing selected".into()) } else { Ok(()) }
+    let st = s.active().ok_or("no document open")?;
+    if st.selection.is_empty() { Err("nothing selected".into()) } else { Ok(()) }
 }
 pub fn has_multi(s: &Session) -> std::result::Result<(), String> {
-    has_doc(s)?;
-    if s.active().unwrap().selection.len() < 2 { Err("select at least two objects".into()) } else { Ok(()) }
+    let st = s.active().ok_or("no document open")?;
+    if st.selection.len() < 2 { Err("select at least two objects".into()) } else { Ok(()) }
 }
 pub fn can_undo(s: &Session) -> std::result::Result<(), String> {
     s.active().filter(|d| !d.history.undo.is_empty()).map(|_| ()).ok_or_else(|| "nothing to undo".into())