Fix stuck drag after OS hand-off withdrawal, split panel_drag_os, doc fixes

Re-checks the physical button before resuming an internal drag and treats a button-up move mid-drag as a release, closing a stale-payload/silent-drop bug; splits the OS hand-off seam into its own TU for the line ceiling; corrects two docs and skips a redundant module-path lookup.

src/app/CMakeLists.txt

@@ -27,6 +27,7 @@ add_library(reaper_reasampler MODULE
     ${REASAMPLER_SRC_DIR}/shell/panel/panel_audition.cpp
     ${REASAMPLER_SRC_DIR}/shell/panel/panel_bank_ops.cpp
     ${REASAMPLER_SRC_DIR}/shell/panel/panel_drag.cpp
+    ${REASAMPLER_SRC_DIR}/shell/panel/panel_drag_os.cpp
     ${REASAMPLER_SRC_DIR}/shell/panel/footer_input.cpp
     ${REASAMPLER_SRC_DIR}/shell/panel/panel_input.cpp
     ${REASAMPLER_SRC_DIR}/shell/panel/panel_layout.cpp