createTimelineFallbackCopy

Description#

Flattens the config's optional label bag into the exact set the template reads, so the template names what it wants rather than indexing a keyed accessor, and the optional-chaining lives in one place instead of at every use site.

groupLabel keeps the band's key as its own last resort: a consumer who clears the formatter should still get a legible header rather than an empty one that leaves the group unnamed.