fix: stop tooltip dedup from hiding distinct events sharing a title

`chime--deduplicate-events-by-title' was keying off the user-facing
title alone, so two real "1:1" entries on different days collapsed to
one and the second one disappeared from the tooltip.

I changed the key to `(marker-file . marker-pos)' — the source heading
identity — so distinct headings keep both copies and recurring expansions
of one heading still fold to the soonest. When marker info is missing
(synthesized test events, edge cases) the key falls back to title, which
preserves the older fixtures the function was originally tested against.

Three new tests cover the regression paths: two distinct headings sharing
a title both survive, one heading expanded into multiple instances still
collapses to the soonest, and a mix of sourced and unsourced events
behaves correctly under both code paths.

0 files changed, 0 insertions, 0 deletions