When the natural merge start (endTxNum minus the largest power-of-two
step span) falls strictly inside an existing visible file, bump it up
to that file's endTxNum so the candidate window cannot straddle.
Without this clip, non-power-of-2 step layouts — such as those produced
by a step-size rebase that turns one step into a non-power-of-2 number
of tx-nums — let the algorithm propose merge windows whose `from` lies
inside a pre-existing file. Downstream staticFilesInRange would then
silently drop that file and emit a merged segment whose name lies about
its coverage, leaving an overlap with the surviving straddler.

Unskip partial_overlap_must_not_be_selected (which now passes), and add
step_rebase_swallow covering the headline case where a single-step file
arriving at N=2_048_000 has natural span 16384 / natural start 2031616
that straddles [2016000, 2032000). After the clip the safe window
{2032000, 2048000} cleanly absorbs 15 trailing files.