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/testkit/src/lib.rs

@@ -11,6 +11,10 @@
 //! - [`geom`]: geometry assertions (approximate equality, curve sampling, Hausdorff distance).
 //!
 //! This crate may only be used as a dev-dependency (enforced by `cargo xtask layers`).
+// Test support only (a dev-dependency of every crate that uses it): a failed setup or assertion
+// must panic, like `assert!`, so the shipped-code ban on panicking (AGENTS.md › Robustness) doesn't
+// apply here.
+#![allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]
 #![forbid(unsafe_code)]
 
 pub mod fixtures;