[DefKorns]

Moves the back-stack protocol out of main.cpp, replaces the global ExitManager with an explicit exit decision, and makes a killed options hand the screen back to the home menu.

Changes

• back_stack.{h,cpp}: pure functions for OM_BACK_STACK (MakeEntry, PopIfLast, Push, BackCommand). Output is byte-identical to the old inline code. The format is unchanged: entries separated by ;, fields by , (commandPath,scriptPath,titleKey[,marker]).
• ApplicationLifecycle replaces ExitManager. Each exit point now calls Decide(Resume | Leave). ResumeUI.sh still runs from a static destructor, after main()'s locals (SDL) are released, and at most once.
• SIGTERM / SIGHUP handler: a killed options (killall, S8103OptionsMenu stop) now resumes the UI and clears /tmp/options.flag. The handler only uses async-signal-safe calls (vfork + execve, then _exit).
• launch_save_manager.sh: on 4-field entries, title_key used to swallow ,marker. The marker is now passed on as --selectedMarker.

Exit decisions

Exit point	Decision
Startup exit(1) (missing or empty commands folder)	Resume
SDL_QUIT, reset button	Resume
A on an external command	RESTART_UI decides (default: Leave)
B with a valid parent entry	Leave (parent menu relaunched)
B at root, or malformed parent entry	Resume
Delete (hold B + confirm)	Leave (DELETE_STR relaunches)
SIGTERM / SIGHUP	Resume, unless a Leave was already decided

RESTART_UI keeps its old meaning. Decide runs before the system() it guards, so a signal during that call does not resume over a screen another process owns.

Testing

• Host (local harness, not part of this PR): the real options binary built natively in Docker, with an LD_PRELOAD shim that records system() calls and injects keys. It pins the exact OM_BACK_STACK and backCommand strings, the early-exit ResumeUI, the external/back/delete paths, the SIGTERM/SIGHUP exit code with a single resume, command-file parsing (CRLF, comments, lines without =), and the launchers run with stubs. All pass.
• Console (SNES Classic, plain build):
  • Startup exits, B at root, B in a submenu, external command, delete and the reset button all behave as in the table.
  • With the menu idle: SIGTERM and SIGHUP resume ReedPlayer (T → running) and remove the flag.
  • With a confirm dialog open: SIGTERM does the same. Before this change it did nothing, because only the main loop reads SDL events.
  • launch_save_manager.sh runs under BusyBox ash with 3- and 4-field entries.
• Baseline check: before this change, SDL's own handler already turned SIGTERM into SDL_QUIT, but only in the main loop. SIGHUP was not handled at all.

Not covered

• SIGKILL and crashes. optiond ignores the exit status, so it cannot recover either.
• Add-on binaries (theme_manager, the gamepad tools) do not get the handler. Their vendored OptionsMenu picks up back_stack only when they bump the submodule.
• The same marker fix for launch_favorites_manager.sh lives in the om-favorites repo and needs its own PR.