When --approve-edits is on, set_pou_code:
1. finds the existing mirror .st for the POU (best-effort glob; no
match -> skip the gate, proceed)
2. composes the proposed merged content (decl/impl swapped at the
IMPL_SENTINEL line, keeping the unchanged half)
3. writes the proposal to <existing>.staged
4. spawns 'phobiCS-tui approve <existing> <staged>' with stdio
inherited so the user sees and answers it
5. cleans up .staged regardless of outcome
Exit 0 -> accepted -> apply the change. Exit 1 -> user-facing
'rejected' response (isError=false; the user actively said no).
Exit 2 -> error response (isError=true).
When the flag is off (default), set_pou_code runs unchanged -- this
keeps existing scripted flows from regressing.
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