Validate package model IDs before submitting jobs

[i386]

Summary

• Reject invalid explicit models package --model-id values while resolving the local package plan.
• Make dry runs and confirmed submissions fail before queuing a Hugging Face Job when the model id is not a model coordinate.
• Add focused tests for explicit valid ids, invalid short ids, and the default derived id path.

Context

While publishing the Gemma layer package from PR #888, job 6a37d21c3093dba73ce2b607 failed remotely because --model-id gemma-4-E4B-it-Q4_K_M passed local planning but was rejected by skippy-model-package write-package inside the paid HF job. This PR moves that validation into package preparation so the mistake is caught during --dry-run and before confirmed submission.

Validation

• cargo fmt --all
• cargo test -p model-package

Protocol

No mesh protocol or package format changes.

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• Commit unit tests in branch jd/package-model-id-validation

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