feat(config-settings): replace the ANSI renderer with a curses front-end

config-settings is now a launcher for scripts/config-settings-tui.py,
drawn with Python's stdlib curses. The seven fish files that hand-rolled
the ANSI renderer are gone, along with the golden harness that had to pin
their byte-exact output.

The TUI is a child process, so it can neither read the session's global
variables nor write them. State goes in as a dump from the new
__config_settings_state; the edits come back as a fish script that
config-settings sources, which is what lets the Session page's `set -g`
land in the caller's shell instead of in a child that is about to exit.
Every edit is emitted as a call to __config_settings_apply or
__config_settings_set_value, so list splitting, the SCROLLBACK_HISTORY_*
export mirror and the shadow-warning suppression all stay in the fish
layer that already owned them.

The consequence, and the one behaviour change: edits are applied in one
batch on exit rather than on each keypress. The status bar shows a
pending count.

New: `/` filters the current page, and on the Universal and Session pages
it reaches into every category's sub-categories, listing hits as
"Category › Sub" so a sub-category can be toggled without drilling into
its parent first. Also a `?` help overlay, mouse selection, and a
drill-down page that leads with the category's own toggle.

Gone with the renderer: the four width tiers, the wrap-aware erase
arithmetic, the stty/dd/od raw key reader, the panel-height bookkeeping
and the hand-written redraw differ. curses owns all of it, and the alt
screen plus absolute addressing makes the desync class behind 608b022,
4210f3b, 93fc5e0 and 3c4f720 unreachable.

The sub-category taxonomy is NOT duplicated in Python: it travels in the
state dump, still sourced from __config_settings_subcats. The category,
Sponge and Paths row tables move into Python, consolidating the two
copies the fish renderers kept.

Dependency: python3 with curses. Stdlib on Arch, Fedora and a full
Debian/Ubuntu python3; python3-minimal alone lacks _curses. The launcher
checks for both and names what is missing. Called out in the README.

Verified: 416/416 assertions, plus a live end-to-end in a sandbox HOME
confirming the Universal page writes universal scope only (U1/G0) and the
Session page global scope only (U0/G1).

README.md

@@ -186,6 +186,8 @@ Then open a new Fish shell — Fisher will be installed automatically on first l
 
 A [chezmoi](https://www.chezmoi.io/) dotfile manager is also configured — secrets are kept in a private overlay directory (see [Personalization](#personalization)) and excluded from version control.
 
+**External requirements.** Everything degrades gracefully if a tool is missing, with two exceptions worth calling out. `python3` is required by the documentation pipeline and by `config-settings`, whose TUI is drawn with the stdlib `curses` module. That module ships with CPython on Arch, Fedora and a full Debian/Ubuntu `python3`; `python3-minimal` alone does **not** carry `_curses`, so install the complete `python3` package there. `config-settings` checks for both and tells you which is missing rather than failing inside the renderer.
+
 > [!IMPORTANT]
 > `config.fish` ends with a `return` sentinel guard. Any lines appended **after** it by a tool's setup command will silently have no effect. Many tools (starship, zoxide, mise, etc.) offer a setup command that appends an `init | source` line to your `config.fish` — all integrations are managed through `conf.d/` files instead. If you add a new tool and its shell integration appears to do nothing, check whether its setup command appended an init line to the bottom of `config.fish` and create a `conf.d/<tool>.fish` file for it instead.
 
@@ -288,7 +290,7 @@ To opt out, set `__fish_user_dots_symlink` to a falsy value (or toggle **Dots li
 
 Everything opinionated in this config — command shadows, startup side-effects, key and environment overrides, terminal integrations, and the first-run greeting — is active by default but can be switched off. Logging (C5) is the exception: it is **opt-in**, off until you explicitly enable it.
 
-> **The easy way — `config-settings`:** Run `config-settings` for an interactive TUI that manages settings across four pages — **Universal** and **Session** (the opinionated category toggles below, persistent or per-shell), **Sponge** (history-scrubbing: delay, exit codes, purge-on-exit, and extra sensitive variable names), and **Paths** (scrollback log dir, max files, and the user-dots path) — without typing a single variable name. Navigate with the arrow keys (or `h`/`j`/`k`/`l`); toggle rows step OFF ← DEFAULT → ON, value rows edit inline with `Enter` and clear with `←`. `Tab`/`Shift-Tab` cycle pages and `q` quits. Changes apply instantly. The panel auto-sizes to your terminal width (four tiers from 52- to 78-wide with a 6-column margin), centers itself horizontally, and redraws within ~0.3 s of a resize.
+> **The easy way — `config-settings`:** Run `config-settings` for an interactive TUI that manages settings across four pages — **Universal** and **Session** (the opinionated category toggles below, persistent or per-shell), **Sponge** (history-scrubbing: delay, exit codes, purge-on-exit, and extra sensitive variable names), and **Paths** (scrollback log dir, max files, and the user-dots path) — without typing a single variable name. Navigate with the arrow keys (or `h`/`j`/`k`/`l`); toggle rows step OFF ← DEFAULT → ON, value rows edit inline with `Enter` and reset with `←`. `Enter` on a category opens its sub-categories, `/` filters the page — reaching into sub-categories too, listed as `Category › Sub` — `?` opens a help overlay, `Tab`/`Shift-Tab` cycle pages, and `q` applies your edits and quits. Edits are collected as you make them and written in one batch on exit; the status bar shows the pending count. The panel is drawn with Python's stdlib `curses`, so it resizes with the terminal and never flickers — see [Installation](#installation) for the `python3` requirement.
 
 If you'd rather set them by hand, each category is controlled by a universal variable. Six category toggles and one master switch are available: