Add a Frontend interface; make the process entry frontend-agnostic.

main.cpp was entirely TUI-specific (constructed Tui, parsed argv, drove
BootDispatch/DumpOutput/Run directly). Introduce a shared frontends layer so a
second frontend can reuse the whole launch flow:

  - src/frontends/frontend.hpp — abstract Frontend interface (BootDispatch,
    DumpCommandsMd, DumpOutput, Run), header-only, no GUI toolkit, no core dep.
  - src/frontends/frontend_main.{hpp,cpp} — frontend_main(argc, argv, Frontend&):
    all the argv parsing (--source/--restore/--batch/--commands-md/--help) and
    the boot → batch/run flow, driving any frontend through the interface.
  - Tui now implements Frontend (the four methods already matched; just marked
    override).
  - The TUI main.cpp shrinks to: construct Tui, call frontend_main. A second
    frontend's main() is identical with its own Frontend type.

Build: a small GUI-toolkit-free static lib essim_frontend (frontend_main.cpp)
is added at the top level when a frontend is selected, and the essim exe links
it. ESSIM_FRONTEND=none still builds core+tests only (no essim_frontend, no
FTXUI). Binary stays ./build/essim.

Behaviour unchanged across --batch/--commands-md/--help/exit codes; only the
usage text is genericised ("the TUI" → "the interface", "console screen" →
"console") now that the launcher is shared.

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

CMakeLists.txt

@@ -50,6 +50,12 @@ if(ESSIM_FRONTEND STREQUAL "none")
   message(STATUS "essim: ESSIM_FRONTEND=none — core + tests only (no frontend, no GUI toolkit)")
 elseif(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/src/frontends/${ESSIM_FRONTEND}/CMakeLists.txt")
   message(STATUS "essim: building frontend '${ESSIM_FRONTEND}'")
+  # Shared, GUI-toolkit-free frontend support: the abstract Frontend interface
+  # (header-only) and the frontend-agnostic launcher frontend_main(). Every
+  # frontend's main() links this and forwards argv to it.
+  add_library(essim_frontend STATIC
+    "${CMAKE_CURRENT_SOURCE_DIR}/src/frontends/frontend_main.cpp")
+  target_include_directories(essim_frontend PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src)
   add_subdirectory(src/frontends/${ESSIM_FRONTEND})
 else()
   message(FATAL_ERROR