Parts that organize build output into `(overlay)` currently accept `overlay-script` and `override-overlay`, allowing conflicting lifecycle configuration. Reject either script during part validation and explain how to generate files in `$CRAFT_PART_INSTALL` during `override-build`, or move overlay modifications to a separate dependent part. The validation uses the existing `organizes_to_overlay` predicate and treats empty scripts as defined, while allowing `None`. Preserve model-level Pydantic errors when converting them to `PartSpecificationError`; otherwise the new actionable message is discarded because these errors have no field location. Fixes #1735. ## Validation - `make test-fast`: 5274 passed, 4 skipped, 1 xfailed; 589 slow tests deselected. - 17 new regression cases cover both script keys, empty scripts, overlay destinations, null scripts, valid non-overlay configurations, and preservation of model-level error messages. - Actual CLI invocation rejects invalid configuration before execution and displays the resolution guidance. - Repository Ruff lint/format and codespell passed; `uv lock --check` and `git diff --check` passed. - Standalone `ty check --python .venv craft_parts tests` completed without errors, with one unrelated unused-ignore warning. The aggregate `make lint` command stopped because the snap-installed type checker could not resolve system APT modules. - Sphinx HTML build with warnings treated as errors and repository RST lint passed. - Vale spelling/inclusive-language checks on the changed documentation report exactly the same 89 findings as the base revision, with no new findings. - Full slow/integration suite was not run. ## Checklist - [x] Have you followed the guidelines for contributing? - [x] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [ ] Have you successfully run `make lint && make test`? - [x] Have you added an entry to the changelog (`docs/reference/changelog.rst`)? The Canonical CLA check passed on this PR.