fix: issues found while enabling `compiler.postponeCompile` for core

This PR fixes a panic in `leanir` and three smaller issues found while enabling `compiler.postponeCompile` for core.

* `leanir` marks its re-seeding of import-derived extension state as unlogged, which otherwise panics for extensions that log writes such as `classExtension`.
* `isDeclMeta` also consults the current module's state for imported declarations.
* `[inherit_doc]` from a private to a public declaration copies the docstring, as importers cannot resolve an alias to a private declaration.
* The IR interpreter throws an exception instead of failing an assertion when asked to evaluate a declaration with an unreachable body.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>