Sort scheduler reconciliation by UTC timestamp

[joeszilagyi]

Fixes #743.

Summary:

• Sort runtime-ledger events and derived outcomes by parsed UTC timestamps instead of raw timestamp strings.
• Add a regression covering offset timestamps that lexical-sort differently from chronological UTC order.

Validation:

• python3 -m py_compile tools/common/scheduler_failure_reconciliation.py tests/test_scheduler_failure_state_reconciliation.py
• /tmp/codex-ruff-venv/bin/ruff format --check tools/common/scheduler_failure_reconciliation.py tests/test_scheduler_failure_state_reconciliation.py
• /tmp/codex-ruff-venv/bin/ruff check tools/common/scheduler_failure_reconciliation.py tests/test_scheduler_failure_state_reconciliation.py
• python3 -m pytest -q tests/test_scheduler_failure_state_reconciliation.py