Define readonly state semantics

[rlippmann]

Explore whether Context Compiler should support authoritative state that cannot be modified by normal directives.

Questions to resolve:

• Is readonly a directive (for example set readonly premise) or state metadata?
• Does readonly apply to premise, policy items, or global state?
• Can readonly be removed?
• What happens when a directive attempts to mutate readonly state?
• Does import/export preserve readonly state?

This is a semantic authority feature, separate from implementation-level mutation hardening.

[rlippmann]

Implementation-level mutation hardening is now tracked separately in #278. This issue should remain scoped to semantic readonly state, as originally intended.