refactor: extract message grouping logic (P3.9)

- Create src/message_grouping.rs module (255 lines)
- Add MessageGroup enum (DateSeparator, SenderHeader, Message)
- Add group_messages() function for date/sender grouping
- Write 5 unit tests (all passing)
- Add full rustdoc documentation with examples
- Update REFACTORING_ROADMAP.md (Priority 3: 3/4 tasks, 75%)
- Update CONTEXT.md with refactoring progress
- Overall refactoring progress: 11/17 tasks (65%)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/lib.rs

@@ -7,6 +7,7 @@ pub mod constants;
 pub mod error;
 pub mod formatting;
 pub mod input;
+pub mod message_grouping;
 pub mod tdlib;
 pub mod types;
 pub mod ui;