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>

apps/vectorcraft/build.rs

@@ -4,12 +4,12 @@
 //! cross-compile from macOS or Linux still links, unless `VECTORCRAFT_REQUIRE_WINRES=1` turns it into
 //! an error (for release builds).
 
-fn main() {
+fn main() -> Result<(), String> {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rerun-if-changed=../../assets/app-icon/vectorcraft.ico");
     println!("cargo:rerun-if-env-changed=VECTORCRAFT_REQUIRE_WINRES");
     if std::env::var("CARGO_CFG_TARGET_OS").as_deref() != Ok("windows") {
-        return;
+        return Ok(());
     }
     let mut res = winresource::WindowsResource::new();
     res.set_icon("../../assets/app-icon/vectorcraft.ico")
@@ -21,8 +21,9 @@ fn main() {
         .set("InternalName", "vectorcraft");
     if let Err(e) = res.compile() {
         if std::env::var_os("VECTORCRAFT_REQUIRE_WINRES").is_some() {
-            panic!("embedding Windows resources failed: {e}");
+            return Err(format!("embedding Windows resources failed: {e}"));
         }
         println!("cargo:warning=vectorcraft.exe built without icon/version resources: {e}");
     }
+    Ok(())
 }