## What `_part_num()` in `imagecraft/pack/grubutil.py` uses `getattr(structure_item, "number", None)` because the `number` field is only defined on `GPTStructureItem`, not the shared `StructureItem` base used across the `StructureList` union (`GPTStructureList | MBRStructureList | HybridStructureList`). Without an explicit annotation, mypy infers `Any` for the `getattr()` result, which trips `no-any-return` when it's returned from a function declared to return `int | None`. ## Why a separate PR This is a pre-existing mypy finding on `main`, unrelated to the `work/starbase/IMAGECRAFT-197` Starbase merge (PR #449) — it was noticed while validating that merge, but the bug and fix exist independently of it. Per the `update-starbase` skill's preparation-PR rules, unrelated pre-existing issues get their own PR rather than being bundled into the merge PR. ## Change Annotate the local variable (`explicit: int | None = ...`) so mypy narrows the `getattr()` result to the function's actual return type instead of inferring `Any`. No behavior change. ## Validation - `make format` - `make lint` (includes mypy) - `make test-fast` 🤖 [BEEP BOOP, A BOT WROTE THIS COMMENT - Claude Sonnet 4.5, GitHub Copilot Chat]