Reconnect to an already-injected DLL (reuse it, survive a tool restart)

Disconnect -> reconnect now reuses the DLL already in the game instead of
injecting again, including across a tool restart or crash: a connected DLL keeps
its per-pid shared section (and worker) alive after the host goes away, so a
fresh host can find it and re-attach to the same section.

- hook_dll_alive(pid) (host/src/inject/dll_probe.cpp): detect a live DLL by
  opening the per-pid section and polling its heartbeat (returns as soon as a
  beat lands; a missing section or stalled worker reads as not-alive). It does
  not check magic -- a graceful disconnect zeroes magic but the DLL keeps
  beating and the worker never re-checks magic post-connect.
- InjectionPanel: the Inject and Connect button branches to reconnect_selected()
  when a live DLL is detected -- IpcServer::start() re-attaches to the SAME
  section the DLL still holds and re-publishes the subsystem state; no
  re-injection. Factored the shared post-connect setup (publish_subsystem_state
  / begin_liveness_tracking). The DLL needed no change -- it just resumes reading
  the re-attached section.
- A false not-alive is benign: the inject path still re-attaches an
  already-injected DLL (LoadLibrary no-ops), so the timeout only needs to clear
  the worker's ~250ms beat period with margin.

Test (mock_game_test test_reconnect): inject -> hooked -> graceful disconnect ->
drop the host handle (simulating a restart while the DLL keeps the section alive)
-> detect via heartbeat -> re-attach to the same section -> hooks re-install
without re-injecting -> and hook_dll_alive goes false once the game is gone.

Roadmap: both current tasks (graceful disconnect, reconnect) done -> removed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/CMakeLists.txt

@@ -221,7 +221,8 @@ add_test(NAME dx12_present_hook_test COMMAND dx12_present_hook_test)
 # stresses hook/unhook cycles. Reuses the shipping shared-texture reader. Skips without D3D11.
 add_executable(mock_game_test
 	mock_game_test.cpp
-	${CMAKE_SOURCE_DIR}/host/src/capture/shared_texture.cpp)
+	${CMAKE_SOURCE_DIR}/host/src/capture/shared_texture.cpp
+	${CMAKE_SOURCE_DIR}/host/src/inject/dll_probe.cpp)
 target_include_directories(mock_game_test PRIVATE
 	${CMAKE_SOURCE_DIR}/host/src
 	${CMAKE_SOURCE_DIR}/tools/mock_game)