fix: honour selected desktop icons in User Menu and Button Bank launches

[midwan]

Summary

Fixes #155.

Functions launched from the User Menu or a Button Bank passed no source lister and no arguments, so entry wildcards like {f} could not see a drive icon selected on the desktop and the command silently did nothing — while the same function worked from the icon context menu.

This adds a launch-time desktop-selection fallback that mirrors the context menu:

• lister_has_selection() — precedence guard. Returns true when any source-eligible lister already has a selection, in which case the desktop is not snapshotted (a lister's selection keeps precedence).
• desktop_selection_argarray() — collects the selected desktop objects into an argument array in the same form the icon context menu supplies them: drives/assigns contribute their device name (e.g. DH0:), left-out icons their name.
• Both launch sites (User Menu in event_loop.c, Button Bank in buttons_run.c) consult the guard and, when no lister provides a selection, launch the function externally with the snapshot and the desktop folder as the source path.

Result: C:List {f} on a selected drive icon now lists that device's contents, as expected.

Product decisions (from the brainstorm plan)

• Behavioural target: match the icon context menu.
• Launch scope: User Menu + Button Bank only (hotkey/ARexx launches are deferred).
• Precedence: desktop is a fallback; a lister's selection wins when present.
• Mechanism: launch-time snapshot (not an engine-level source-resolution change).

Plan: docs/plans/2026-08-03-001-fix-desktop-icon-selection-plan.md (requirements R1–R6; the plans directory is gitignored).

Testing

• New static test source/Program/tests/test_desktop_selection_fallback.py (3 tests), written red-first, now green.
• Full static suite: 113 passed, no regressions.
• Builds verified for os3, os4, mos, i386-aros, x86_64-aros. (The local ppc-morphos tag image is the broken 2026-07-04 rebuild, so MorphOS was built with the known-good digest pinned in CI.)
• No new compiler warnings introduced (the remaining function_launch.c warning is pre-existing, in untouched code).

Runtime acceptance (manual — not covered by CI)

On a built AmigaOS target:

• Select a drive icon on the desktop, run C:List {f} from the User Menu (output to window) → a shell lists the device contents.
• With files selected in a lister, run the same from a button → the lister selection is used, not the drive.
• Lister open but nothing selected → falls back to the drive icon.
• The drive icon remains selected after the function runs.
• A selected left-out icon is honoured too.

Notes

• source/Program/tests/__pycache__/ is generated by pytest and is intentionally not committed; consider adding __pycache__/ to .gitignore as a follow-up.