Enforce deep immutability across public TRIA values

[SarashaElion]

Summary

Build 024 hardens TRIA's immutability guarantees so frozen public value objects cannot still be mutated through nested dict/list containers.

Event integrity

• EventProposal and RelationalEvent deep-freeze payloads at construction
• committed event payloads become immutable snapshots of caller-owned data
• later mutation of the source object cannot alter stored event meaning
• serialization thaws frozen containers back to ordinary JSON-compatible structures
• canonical event hash semantics remain unchanged

Runtime / provider / governance boundaries

• invocation request/result metadata is deeply immutable
• context values and runtime tuple surfaces are normalized at construction
• provider request payloads and metadata are deeply immutable
• provider response metadata is deeply immutable
• governance decision metadata is deeply immutable

Conformance

• tests prove source-object mutation cannot alter TRIA values after construction
• tests prove nested writes fail
• adds deep-immutability documentation and conformance fixture
• adds deep_immutable_public_values to the conformance manifest

Architectural intent

Immutability is a structural property, not merely @dataclass(frozen=True). A value that can be changed through a nested container is not actually immutable.

This is representation hardening only. It does not change the event schema, projection version, bundle format, or historical event hash semantics.