Text rules: auto-capitalization, double-space period, backspace repeat

- TextContext: field purpose/hints and text before the cursor, reported by
  the backend (KWin content_type + surrounding_text)
- TextRules: sentence-start and double-space heuristics, deletion helpers
- KeyRepeater: held backspace repeats, then deletes whole words (Ctrl+BS)
- Controller keeps a best-guess of the text before the cursor so rules work
  instantly and in fields that don't report surrounding text
- No auto-capitalization or smart punctuation in password/URL/email fields

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CMakeLists.txt

@@ -2,11 +2,14 @@
 
 qt_add_library(sloposk_core STATIC
     backend/TextBackend.h
+    backend/TextContext.h
     engine/InputEngine.h
     engine/DirectEngine.h engine/DirectEngine.cpp
     input/ShiftState.h input/ShiftState.cpp
     input/PointerTracker.h input/PointerTracker.cpp
     input/AlternatesPicker.h input/AlternatesPicker.cpp
+    input/KeyRepeater.h input/KeyRepeater.cpp
+    input/TextRules.h input/TextRules.cpp
     input/KeyboardController.h input/KeyboardController.cpp
     input/KeyboardSurface.h input/KeyboardSurface.cpp
     layout/Layout.h