Fix latest session selection by start time

[grp06]

Summary

• Sort TraceStore sessions by started_at descending with a session ID tie-breaker.
• Add a latest-session metadata helper and use it for freshness checks.
• Update the storage ADR and add regression coverage for chronological ordering plus malformed metadata skipping.

Tests

• PYTHONPATH=src python -m unittest tests.test_store tests.test_freshness -v
• PYTHONPATH=src python -m unittest discover -s tests -v
• Manual proof script for opposing session ID and timestamp ordering