createSignificantChangeTracker

Description#

Derive the most recent significant transition from a chart-summary signal, or null when the latest update carried no significance.

Mirrors createTransitionTracker's current/previous discipline: an internal linkedSignal holds the current summary and the one before it (structural-equality guarded), and the public signal is a computed with its own structural equal - so a summary that re-emits an equivalent shape yields the same null reference and never re-fires a downstream effect. Threshold crossings take precedence over trend flips (they are operationally more significant).

The chart stays pure-derivation: effects that react to the event live in the companion announcer, not here.