Normalize system keyboard shortcuts: program-stop, IDE exit, strict modifier matching

[dsent]

Summary

Rework the IDE's platform-level (system) keyboard shortcuts so the "get out"
keys follow familiar console conventions, an accidental extra modifier can't
misfire a system chord, and exiting the whole IDE is a single deliberate
action. Editor-internal shortcuts are out of scope here.

Motivation

The current system chords overload keys with meanings that fight muscle
memory, and some can fire when extra modifiers are held:

• Ctrl+S currently stops a running program (returns to the console).
  Everywhere else Ctrl+S means "save," so using it to stop/exit surprises
  anyone with desktop habits. Moreover, we're teaching the kids wrong
  conventions they'll have to relearn when moving on to PCs.
• Exiting the whole IDE is context-dependent today (Ctrl+Esc behaves
  differently inside a program vs. at the console), so "how do I fully exit"
  has no single answer. Ctrl+Esc duplicates another shortcut (currently,
  Ctrl+S).
• System chords can be triggered by a superset combo (e.g. Ctrl+Shift+Esc
  firing the Ctrl+Esc action), which makes fumbles hazardous.

The audience is children, so the system-chord set should be small,
predictable, convention-aligned, and hard to trigger destructively by accident.

Proposed changes

1. Stop the running program: Ctrl+S → Ctrl+D
Ctrl+D is the console EOF / "I'm done" convention for leaving an interactive
session. Rebind the existing "stop the running program, return to the console"
action from Ctrl+S to Ctrl+D. Ctrl+C (the other console convention)
stays as copy. Ctrl+S is left with no system binding.

2. Exit the whole IDE: Ctrl+Alt+Esc, always
Make "exit the whole IDE" a single, context-independent action bound to
Ctrl+Alt+Esc — it exits from inside a running program and from the console
alike. Ctrl+Alt+* is the adult/teacher-oriented modifier family, so a
destructive whole-IDE exit sits behind a deliberate 3-key combo a child won't
hit by accident. Bare Ctrl+Esc becomes unbound.

3. Strict modifier matching for system chords
A system shortcut fires only when the pressed modifier set matches exactly.
Extra held modifiers suppress it — e.g. Ctrl+Alt+Shift+Esc must not still
fire the Ctrl+Alt+Esc exit, and no superset combo triggers a subset chord.

4. Drop "duplicate line"
Remove the Ctrl+D "duplicate current line" action in the input strip.
Select + copy + paste already covers it; removing it simplifies the input
layer and frees Ctrl+D for the stop-program role above.

Resulting system-chord map

Key	Action
Ctrl+C	copy
Ctrl+D	stop running program → console
Ctrl+Q	quit project
Ctrl+Alt+Esc	exit the whole IDE (from anywhere)
Ctrl+Esc	(unbound)
Ctrl+S	(unbound)

Notes

• Editor-internal shortcuts (in-editor navigation, checkpoint/restore, leaving
  the editor) are handled separately and are out of scope for this issue.

[dsent]

@nagydani what do you think?

[dsent]

No objections from Dani after a private conversation.