[Feat] Package/registry for policy packs + stdlib modules

[ax1s-x1zz]

Context

docs/ROADMAP.md Track E-4. Three domain policy packs (healthcare/finance/public) exist; a sharing mechanism turns them into an ecosystem.

Scope

• Registry format for policy packs and stdlib modules
• xazz policy install <pack> / xazz std install <module>

Dependencies

Depends on B-2 (stdlib) and the module system.

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See docs/ROADMAP.md for the full scale plan and execution order.