Overhaul packer workflows and GUI

[NathanNeurotic]

Summary

• add a cross-platform CI workflow plus a manual artifact builder that packages binaries with README and LICENSE
• refactor psu-packer into a reusable library with strict psu.toml validation, timestamp policy overrides, verification, and golden fixture tests
• replace the legacy suitcase UI with a minimal validation/export flow backed by the new packer core and document the schema/templates

Testing

• cargo test

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https://chatgpt.com/codex/tasks/task_e_6901d339e8bc83219265fdf17234d403