fix(pen): re-baseline zoom on pointer-count change; observe pen on child HWND

Zoom jumping (device: min 0.5 / max 2.47 while zooming near 1): the per-frame
scale clamp limited single-frame spikes but not multi-frame runs. Root cause is
pointer-count transitions — on Windows touch the two fingers land/lift at
different times and digitizers drop/re-acquire touches, and stock InteractiveViewer
keeps _scaleStart/_referenceFocalPoint from the OLD finger set, so the next frame
jumps. PenInteractiveViewer now re-baselines (and skips the transitional frame)
whenever details.pointerCount changes. The per-frame clamp stays as a secondary
guard.

Buttons (device evidence: btn=1 for tip-down, side-button, AND inverted; kind
never becomes invertedStylus): Flutter does NOT surface the barrel/eraser/inverted
state at all — unreachable from Dart. The only path is the native badnote/pen
plugin, which was SILENT because WM_POINTER is delivered to the Flutter CHILD
view window, not the top-level FlutterWindow where ObservePenMessage was hooked.
Fix: subclass the child HWND (SetWindowSubclass + comctl32) and observe its
WM_POINTER messages, passing every message through unchanged via DefSubclassProc
(observation-only, input behavior preserved). This is what should finally feed
GetPointerPenInfo penFlags + tilt to the channel — to be confirmed on-device with
the diagnostic (btn / kind / tilt readout).

Dart: analyze clean, 66/66 tests, linux build green. Native compiles on Windows CI.

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

windows/runner/CMakeLists.txt

@@ -36,6 +36,9 @@ target_compile_definitions(${BINARY_NAME} PRIVATE "NOMINMAX")
 # dependencies here.
 target_link_libraries(${BINARY_NAME} PRIVATE flutter flutter_wrapper_app)
 target_link_libraries(${BINARY_NAME} PRIVATE "dwmapi.lib" "windowsapp.lib")
+# comctl32 provides SetWindowSubclass/DefSubclassProc for the pen WM_POINTER
+# observer installed on the Flutter child window (flutter_window.cpp).
+target_link_libraries(${BINARY_NAME} PRIVATE "comctl32.lib")
 target_compile_options(${BINARY_NAME} PRIVATE /await)
 target_include_directories(${BINARY_NAME} PRIVATE "${CMAKE_SOURCE_DIR}")