Refactor temporal_buffers_test.py: reduce duplication with fixtures and helpers

- Add fixtures: single_slice_2element, thick_slice_2x2 for common test data patterns
- Add helper functions: make_single_slice(), make_thick_slice(), assert_buffer_has_time_data()
- Parametrize test_add_data_creates_time_dimension to cover single and thick slices
- Remove private state testing from TestTemporalBuffer:
  - Delete test_set_required_timespan (was testing _required_timespan)
  - Delete test_set_max_memory (was testing _max_memory)
  - Delete test_max_memory_limits_capacity (was testing _data_buffer.max_capacity)
- Rename remaining setter tests to reflect behavior-based testing intent
- Simplify all buffer creation code using make_single_slice and make_thick_slice helpers
- Reduce test file from 730 to 576 lines while maintaining full coverage

Tests focus now on observable behavior (data being trimmed, concatenated, etc.) rather
than inspecting implementation details. This makes tests more resilient to internal refactoring.

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Author: SimonHeybrock