MAINT: Test and clarify timing precision accessors

[lntutor]

Summary

• add isolated coverage for timing precision reads, updates, and validation errors
• clarify get_default_precision() as the read-only equivalent of float_precision()
• replace uncollected doctest-style prompts with regular usage guidance
• clean existing whitespace violations in the timing module

Closes #889

Validation

• uv run --extra testing pytest quantecon/tests/test_timings.py -q
• uv run --extra testing flake8 quantecon/timings quantecon/tests/test_timings.py

I also started the full pytest quantecon -q suite; it ran cleanly through 47% before the local terminal session ended unexpectedly.