fix: reject empty kernel packages before linking

[KitaroMoha563]

Describe your changes

Reject kernel packages whose manifest does not export any $kernel procedures before converting them into a runtime Kernel.

This turns malformed package metadata into a structured package error before it reaches the linker's non-empty-kernel assertion. Assemblers without a kernel still use the existing empty-kernel default.

Tests cover both direct package conversion and Assembler::link_package() returning an error instead of unwinding.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Commits are signed.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.
• Updated CHANGELOG.md

[github-actions]

Automated check (CONTRIBUTING.md)

Findings:

• Link an issue in the PR body (e.g., "Fixes Implement SHA256 in Miden Assembly #123").
• Add a short Rationale explaining why the change is needed.

Recommendations:

• Consider adding a Test plan or clear review steps.

Next steps:

• Link a relevant issue (e.g., "Fixes Implement SHA256 in Miden Assembly #123") and ensure it is assigned to you.
• See CONTRIBUTING.md for expectations.
• If this is a false positive, comment: /quality-review.

[huitseeker]

Thanks for the PR @KitaroMoha563 ! This LGTM.

Note for @bitwalker : @bobbinth had a comment suggesting kernel procedure digests might be derived from ModuleInfo rather than the package manifest. This PR goes a bit further in the current implementation’s direction instead.