Add scripts/config-settings-tui.py, a stdlib-curses prototype of the
config-settings interface, plus a gate that runs its --self-test.
The backend is stubbed: values live in an in-memory dict and nothing is
read from or written to fish variables. This is here to evaluate the
render engine and the interaction model before committing to a rewrite.
Why curses rather than more ANSI arithmetic:
- No flicker, structurally. curses diffs its virtual screen against the
physical one and emits only the changed cells, which is what
__config_settings_diff_redraw.fish reimplements by hand.
- Alternate screen plus absolute addressing. Stray output cannot desync
the display, so the bug class behind 608b022 (fish's read prompt),
4210f3b (a shadow warning on stderr), 93fc5e0 and 3c4f720 (line wrap
breaking the erase height) cannot occur at all.
- Resize is a repaint rather than wrap-factor arithmetic.
- Overlays, panes, live filtering and mouse input cost a few lines each.
The layout departs from the current single panel: a page sidebar with a
live filter on the left, a scrolling detail pane on the right, a help
overlay on '?', and mouse selection. Sub-category drill-down, the
tri-state badges and the Sponge/Paths value rows all carry over.
Dependency note: python3 with the curses module. That is stdlib on Arch,
Fedora and a full Debian/Ubuntu python3; python3-minimal alone does not
carry _curses, so the test asserts the import.
The prototype is not wired into config-settings and nothing existing
changed.
