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 <[email protected]>

.github/pull_request_template.md

@@ -0,0 +1,17 @@
+## What and why
+
+<!-- What does this change, and why? Link the issue if there is one. -->
+
+## How it was tested
+
+<!-- Tests added, and how you checked the behaviour (CLI, MCP, screenshots). -->
+
+## Checklist
+
+- [ ] `cargo xtask ci` passes (fmt, clippy, tests, assets, layers, wasm).
+- [ ] No panics in shipped code: no `unwrap()`, `expect()`, `panic!`, `unreachable!`, `todo!` or
+      `unimplemented!` outside tests, and no unchecked indexing on data from files, parameters or
+      messages. Errors are returned as `Result` and shown to the user
+      ([why and how](https://github.com/storytold/vectorcraft/blob/main/docs/development.md#robustness-vectorcraft-never-crashes)).
+- [ ] Clean-room: nothing copied from Adobe products or GPL/AGPL code ([AGENTS.md](https://github.com/storytold/vectorcraft/blob/main/AGENTS.md)).
+- [ ] Every new asset has a row in `ASSETS.md`.