[1/3] core: make world state snapshots serializable

[sayan-oai]

Why

WorldState currently keeps its diff baseline as live Rust objects keyed by process-local TypeId. That baseline cannot be written to a rollout or restored after resume, so Codex reconstructs an approximation from TurnContextItem.

This is the first change in the WorldState persistence stack. It gives every section a stable persisted identity and a compact serializable comparison snapshot without changing rollout behavior yet.

What changed

• Require each WorldStateSection to define a stable ID and serializable snapshot type.
• Reject duplicate section IDs when constructing WorldState.
• Persist a dedicated environment comparison snapshot using model-visible strings instead of runtime path types.
• Store only WorldStateSnapshot in ContextManager, removing the parallel live-object baseline.
• Render diffs by restoring each section's typed snapshot; invalid snapshots fall back to a full section render.
• Omit null object fields for future RFC 7386 patches while preserving null values inside arrays.

Follow-up PRs will record full snapshots and merge patches, then restore the baseline during resume, fork, and rollback.

Test plan

• WorldState snapshot tests cover stable IDs, duplicate rejection, null omission, and array preservation.
• Environment tests cover persistence-safe snapshot values and existing diff rendering.
• ContextManager baseline deduplication and session context-update persistence tests.

Related: #29249