Finish Phase 1 items 1, 2, 4, on top of the range-handling done in 9b78219:
- TextField.Draw now emits, when focused:
* key.InputHintOp{HintText} (item 4: enable text keyboard/autocorrect)
* key.SnippetCmd (the visible window as the snippet, Range {0,len})
(item 2: swipe/autocorrect source)
* key.SelectionCmd (caret, window-relative rune index)
(item 1: IME selection sync)
The snippet is the visible window (not the whole file), so the IME treats
the window as the document and reports EditEvent.Range window-relative.
- HandleReplaceRange now resolves the window-relative range against
IMEWindowText and offsets by IMEWindowStartByte to address the buffer
(string and chunked paths). Falls back to the whole buffer when layout
has not set the window (tests).
- EditorState gains IMEWindowStartByte / IMEWindowText, set during layout.
- Add runeCount helper (utf8 leading-byte scan) in the ui package.
- Fix a data race in three editor e2e/integration tests: they called
OpenFile from the test goroutine while the logic goroutine ran layout;
now wrapped in withState so the state write happens on the owner.
Tests: ime_range_test.go gains windowed-path coverage (string+chunked).
go build, go test, go vet, and go test -race are all green.
Greg Pomerantz
72b3c3f9c1