Stop stepping past end of completed program

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Fixes #109

Problem

Stepping is done via the simulator's debugExecStep, which executes with debugging mode set to true and bypasses the codeRunning check. Because of this, the program end (BRK) was never detected while single-stepping: debugExecStep even cleared the _programCompleted flag and kept executing, so the user could silently step past the end of the program into uninitialized memory.

Changes

Core (packages/6502)

• BRK and unknown opcodes now explicitly mark the program as completed, so completion is detected in debug mode too — stepping over the final BRK now logs Program completed at PC=$… and transitions to COMPLETED state.
• debugExecStep refuses to step a completed program and emits an info message (Program completed. Reset to step through the program again.) instead of clearing the flag. This protects all platforms, including the web app which uses the simulator directly.
• The state getter gives COMPLETED precedence over the stepper flag so the UI reflects completion while debugging.

common-ui

• The step action is disabled in COMPLETED state; the user has to reset or run again (the main button switches to Reset).
• The event bridge dispatches a program-completed notification when execution finishes, prompting users who are watching the GUI debugger.

Apps

• GNOME and Android show a Program completed toast for the new notification key.

Translations

• New strings extracted to the pot and translated in all 15 languages.

Verification

• yarn format → yarn check:format → yarn build all pass.
• Runtime-verified against the built core package: stepping over BRK → COMPLETED + stop event; a further step is refused (PC unchanged) with the info message; after reset, stepping works again; normal run mode still ends in COMPLETED as before.