* 3.6.6 prerelease = false

* Fix multi-sport activity import (#289)

Multi-sport activities (triathlon / brick sessions) are stored by Garmin as
a single FIT file containing multiple `session` messages plus child sessions
for transitions. Previously the importer wrote every session under the bare
parent activity_id, so the last session silently overwrote all the others
and laps/records/HR zones/devices ended up attached to the wrong activity
(or dropped entirely when the hr_zones_timer processing order created empty
rows that blocked the real lap writes).

This change detects multi-sport files by checking `len(fit_file.session) > 1`
and, when detected:

- creates one activity row per session with a suffixed id `<parent>_<n>`
  (e.g. 22638574127_1 ... _5 for swim→T1→run→T2→swim)
- derives each session's `stop_time` from `start_time + total_elapsed_time`
  (Garmin reuses the parent start_time as every child's `timestamp`, which
  would otherwise result in stop_time <= start_time)
- partitions laps by session using each session's `num_laps` and rewrites
  `lap_num` to be session-local (0..N per session), so every child has its
  own contiguous lap sequence
- partitions records (GPS trackpoints) by matching their timestamp against
  each session's time window
- remaps session/lap HR-zone writes to the correct child activity_id
- associates device entries with every child activity

`_write_lap_entry` was also switched to `ActivityLaps.s_insert_or_update` so
lap data correctly coalesces with the hr_zones_timer rows that the generic
message dispatcher creates earlier in the import; the same `stop_time`
correction applies when the file is multi-sport.

Single-session FIT files go through the exact same code path as before.

Extends and supersedes the single-session subset fixed in #310 (same root
cause for `stop_time`, lap-ordering race, and the add-vs-insert-or-update
issue).

Fixes #289

---------

Co-authored-by: Tom Goetz <tcgoetz@gmail.com>
Co-authored-by: Tom Goetz <15090876+tcgoetz@users.noreply.github.com>