Fix max-age clade splitting in as_daisie_datatable()

[joshwlambert]

This PR addresses #60.

A as_daisie_datatable() bug mishandled island colonists with branching times older than island_age in two ways:

1. The splitting loop's exit condition (while (brts[1] >= island_age)) never re-evaluated, so it relied on the loop body emptying event_times to break. Every event in event_times — including the colonisation time and valid in-island branching times — was emitted as its own *_MaxAge singleton row.
2. The colonisation time at event_times[1] was being written into the first split row, producing a Branching_times value older than island_age for that singleton (the symptom reported in Bug in as_daisie_datatable() when branching times older than island age #60).

Changes

For an island colonist where one or more branching times exceed island_age:

• each over-island branching time is split off as its own *_MaxAge singleton row, with the clade name suffixed _1, _2, ...;
• the colonisation time, together with any branching times younger than island_age (i.e. valid in-island cladogenesis), stays as the main *_MaxAge row under the original clade name.

If every branching time exceeds island_age, the main row contains just the colonisation time. Numeric clamping to island_age - epss continues to happen in create_daisie_data(), as before.

• Switched the splitting loop to walk event_times[2] so the colonisation time at index 1 is preserved.
• Re-evaluated the loop condition each iteration (event_times[2] >= island_age), guarded by length(event_times) >= 2 so the loop exits cleanly once only the col_time remains.
• Re-flag the leftover main row as *_MaxAge when its largest event still exceeds island_age.
• Added a @details section documenting the _MaxAge splitting semantics.
• tests/testthat/test-as_daisie_datatable.R: updated three existing test expectations to reflect the corrected splitting; and added a regression test using the exact reproducer from Bug in as_daisie_datatable() when branching times older than island age #60.
  • NEWS.md: bug-fix entry under the development version section.