Harden deferred language surface (export trait, generics, idents, @policy)

[sujaydavalgi]

Summary

• export trait / public trait: traits export across modules for impl (private stays local); Rust + TS module registry.
• Generics hardening (still Experimental): reject empty <>, duplicates, T: Bound, and where with clear errors.
• Typed config/format idents: provider: mock and serialize(x, json) accepted alongside strings; runtime resolves bare format idents.
• @policy(kind: "homeostasis") PoC: attribute form parses (Rust + TS); legacy homeostasis_policy kept with legacy_syntax + lint.

Also rebased onto main after #58 (follow(path:) clamp).

Test plan

• cargo test -p spanda-core --test export_trait_generics
• npx tsc --noEmit
• CI Fast green
• Spot-check: export trait cross-module impl; @policy(kind: "homeostasis") parse; bare provider: mock

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