refactor(console): phase 4 — extract ConsoleSelectionController from the god-class

Pull the text-selection concern out of ConsoleTab into a focused, ImGui-free
ConsoleSelectionController (console_selection_controller.{h,cpp}). It owns the
anchor/caret state and all the fiddly logic around it: the mouse-drag lifecycle
(beginDrag/updateDrag/endDrag), select-all, range ordering, text extraction over
the visible model, and the index shift applied when the buffer cap evicts lines
from the top.

ConsoleTab now delegates: the four scattered selection fields (is_selecting_,
has_selection_, sel_anchor_, sel_end_) and five helper methods (selectionStart/
End, isPosBeforeOrEqual, getSelectedText, clearSelection) collapse to a single
selection_ member plus a thin selectedText() wrapper. screenToTextPos (still
view-side — it needs the frame's layout) now returns the shared ConsoleTextPos,
so ConsoleTab::TextPos is retired.

The byte-offset / eviction math was previously only reachable through the live
ImGui renderer; it now has direct unit coverage (testConsoleSelectionController:
ordering, drag lifecycle, upward drag, select-all, partial/full/no-op eviction
shift, and filtered extraction). Full-node + lite build clean; ctest green;
source hygiene clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -460,6 +460,7 @@ set(APP_SOURCES
     src/ui/windows/console_input_model.cpp
     src/ui/windows/console_model.cpp
     src/ui/windows/console_output_model.cpp
+    src/ui/windows/console_selection_controller.cpp
     src/ui/windows/console_tab_helpers.cpp
     src/ui/windows/console_text_layout.cpp
     src/ui/windows/settings_window.cpp
@@ -585,6 +586,7 @@ set(APP_HEADERS
     src/ui/windows/console_input_model.h
     src/ui/windows/console_model.h
     src/ui/windows/console_output_model.h
+    src/ui/windows/console_selection_controller.h
     src/ui/windows/console_tab.h
     src/ui/windows/console_tab_helpers.h
     src/ui/windows/settings_window.h
@@ -1023,6 +1025,7 @@ if(BUILD_TESTING)
         src/ui/windows/console_input_model.cpp
         src/ui/windows/console_model.cpp
         src/ui/windows/console_output_model.cpp
+        src/ui/windows/console_selection_controller.cpp
         src/ui/windows/console_tab_helpers.cpp
         src/ui/windows/console_text_layout.cpp
         src/ui/windows/mining_benchmark.cpp