TimelineControls: empty render when neither flag is set, reset button
gated on isZoomed, clear button gated on hasSelection, both-on, both
callback wirings.
TimelineXAxis: empty filled → no ticks, populated → ticks render,
omit-year branch when all buckets share a year, show-year branch
across multiple years, length-4 bucket-string fallback.
11 tests across two timeline primitives.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
