fix(shared-state): remove race in dirty-read paths

[SH20RAJ]

Summary

• fix race-prone dirty-read pattern in DirtyVec::read_if_dirty
• fix same race-prone pattern in DirtyValue::read_if_dirty
• fix same race-prone pattern in ChatState::read_if_dirty

Technical change

• take read lock first
• check dirty flag
• clone snapshot while lock is held
• clear dirty flag after snapshot is captured

This ordering avoids clearing dirty before data is observed, which could otherwise cause missed notifications under concurrent writers.

Issue

Fixes #22

Validation

I could not run cargo test in this environment because cargo is unavailable in PATH; the fix is localized to dirty-read methods and preserves existing public method signatures.

[SH20RAJ]

Quick follow-up: this patch changes dirty-read ordering to snapshot under read lock before clearing dirty flag in DirtyVec, DirtyValue, and ChatState. This targets missed-notification races under concurrent writers while preserving external method signatures.