Fix #1000: [Bug]: sort_by: 'insert_at_end' works unexpectedly

[victorwon2001]

Fixes #1000

Implementation summary

insert_at_end now preserves previously rendered buffer order and appends unseen buffers, with deterministic ID ordering for multiple new buffers. Added regression coverage for lower-numbered buffers across redraws.

Changes

lua/bufferline/sorters.lua | 15 ++++++++++-----
 tests/sorters_spec.lua     | 29 +++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 5 deletions(-)

Testing

Yes. Relevant repository checks passed.