Fix mixed-unit session activity ordering

[ScriptedAlchemy]

Summary

• normalize seconds and millisecond session message timestamps before choosing latest scheduler activity
• preserve bounded indexed lookups by comparing the newest raw timestamp from each unit bucket
• add a regression for mixed seconds/milliseconds where the newer seconds timestamp must win

Addresses review thread PRRT_kwDOSzKG2s6N1M6A from PR #218.

Tests

• TMPDIR=/home/zack/.cache/tmp-session-activity-normalized CARGO_TARGET_DIR=/home/zack/.cache/tracedecay-target-session-activity-normalized cargo test --test automation_runner_test load_session_activity_selects_newest_after_normalizing_mixed_timestamp_units
• TMPDIR=/home/zack/.cache/tmp-session-activity-normalized CARGO_TARGET_DIR=/home/zack/.cache/tracedecay-target-session-activity-normalized cargo test --test automation_runner_test load_session_activity
• TMPDIR=/home/zack/.cache/tmp-session-activity-normalized CARGO_TARGET_DIR=/home/zack/.cache/tracedecay-target-session-activity-normalized cargo test --test automation_runner_test scheduler
• TMPDIR=/home/zack/.cache/tmp-session-activity-normalized CARGO_TARGET_DIR=/home/zack/.cache/tracedecay-target-session-activity-normalized cargo check
• cargo fmt --all -- --check

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5bbbffb

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR