Beta blocker: a stub-then-real script set silently replaces the real flow with its placeholder after fmt --upgrade

[ako]

On CapTrackV6: fmt --upgrade -p judges each file against the stored model. It declines the header on 13-actions (the stub of ACT_Export_Excel, MDL-V1-REBUILD) and adds it to 30-export (the real flow).

On exec, 13 (mdl 0) rebuilds the stored real flow into the stub, then 30 (mdl 1) is refused (G2, #897 item 4: a return inside an error handler is not spliced). This repeats on every run: the Excel export stays the "Der Excel-Export folgt." placeholder, mx check reports 0 errors, and only the exec error shows it. That is a valid model running the wrong app.

Fix either or both:

1. G2: splice a fragment whose custom error handler ends in its own return (create or modify flow: remaining splice limits after #886/#888 #897 item 4; repro grow-flow-error-handler-return.*).
2. Detect stub-then-real pairs across a script set: two create or modify statements for one flow in different files of one run. Have fmt --upgrade treat them together (header on both or neither), and have check -p over several files warn. Since A recursive microflow cannot be created in one statement (exec: microflow not found; check passes) #843 a recursive flow needs no stub, so the warning can say so.

Repro: repros3/stub-then-real-loses-flow.sh.

Found by acceptance rehearsal 3 (frozen build e8a5377, five projects; /home/vscode/rehearsal/REPORT3.md). Tracking #714.

[ako]

Fixed by #909 and #908 (both merged):

• fix(flow-modify): splice a new activity whose error handler ends in its own return (#905 part 1, G2) #909, the cause: a new activity whose custom error handler ends in its own return is spliced in under mdl 1 (create or modify flow: remaining splice limits after #886/#888 #897 item 4). On a CapTrack copy, ACT_Export_Excel ends as the real export flow, a second run writes nothing, and mx check matches the baseline.
• check/fmt: read several files as one script set; report and pair stub-then-real flows (#905 part 2) #908, the guard: check over several files warns MDL-STUB01 when one flow is declared twice in the set, and fmt --upgrade over several files gives both files of such a pair the same header.