fix: harden IME selection handling for WebView

[zefutoff]

Improve IME behavior with editors exposing slow or unstable InputConnection selection state, especially Android WebView.

Changes:

• add a debug-only local WebView compatibility fixture;
• move selected-text InputConnection reads off the IME UI thread;
• use EditorInfo initial selection where available;
• track editor selection updates explicitly;
• restore the analyzed selection range before commitText();
• reject stale selection ranges before applying corrections;
• make the late-selection E2E test synchronize on stable IME visibility rather than transient UI state;
• document WebView compatibility testing.

Validated locally with:

• make check
• make assemble
• targeted ImeInputConnectionEndToEndTest
• full :app:connectedDebugAndroidTest
• Samsung Galaxy S21 / Android 15
• Android System WebView 154.0.8037.57

Manual WebView validation:

• IME remains responsive;
• selected text can be analyzed;
• corrected text is committed successfully;
• stale selections are not blindly reused.