The IME commit path (swipe-to-type, autocorrect replacement) sends a
key.EditEvent with a Range that the editor ignored, causing the replaced
region to be duplicated. Add Logic.HandleReplaceRange which deletes the
rune range [start,end) and inserts text, converting rune indices to byte
offsets (string path: utf8.DecodeRuneInString; chunked path: leading-byte
scan). HandleKeyDown now routes key.EditEvent through it.
Also fixes two pre-existing ChunkedBuffer correctness bugs the tests
exposed:
- Delete mis-computed the per-chunk end using a shrinking 'remaining'
instead of the absolute end, corrupting multi-chunk deletes.
- FullContent derived its chunk bound solely from fileLen, truncating
in-memory chunks grown past the old bound by an insert.
Adds ime_range_test.go covering insert/replace/delete, unicode, chunked,
and swapped bounds. go test -race ./... green.
