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).

functions/__config_settings_state.fish

@@ -0,0 +1,76 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# DEPENDENCIES
+#   __config_settings_get_val, __config_settings_get_raw,
+#   __config_settings_subcats
+#
+# SYNOPSIS
+#   __config_settings_state
+#
+# DESCRIPTION
+#   Dumps everything scripts/config-settings-tui.py needs to render the
+#   settings TUI: the current value of every opinionated-component,
+#   sponge, scrollback and user-dots variable, plus the sub-category
+#   taxonomy from __config_settings_subcats.
+#
+#   Two record types, separated by RS (0x1e), fields separated by US
+#   (0x1f). Both are ASCII control characters, so no value these variables
+#   can hold needs escaping on the way through:
+#
+#     var<US><scope><US><name><US><value>
+#     sub<US><category_var><US><slug><US><label><US><description>
+#
+#   Only variables that are actually set are emitted; the TUI renders an
+#   absent variable as DEFAULT. That is what keeps the variable list out of
+#   this function -- names are discovered with `set --names` and filtered by
+#   prefix, so a new sub-category needs no edit here, only in
+#   __config_settings_subcats.
+#
+#   Toggle variables are dumped once per scope (universal and session), since
+#   the TUI edits those scopes independently. Sponge and Paths variables are
+#   universal-only and dumped as the value the running shell resolves,
+#   space-joined for list variables.
+#
+# EXIT STATUS
+#   0  Always
+#
+# RETURNS
+#   The RS/US-separated state dump, printed to stdout
+#
+# EXAMPLE
+#   __config_settings_state | string split \x1e
+function __config_settings_state --description 'Dump config-settings state for the curses TUI'
+    # ── Variable values ───────────────────────────────────────────────────
+    for name in (set --names)
+        switch $name
+            case '__fish_config_op_registry_*'
+                # conf.d data table, not a setting -- shares the op_ prefix.
+                continue
+            case '__fish_config_op_*' __fish_config_opinionated
+                # Toggles: both scopes, independently editable.
+                for scope in universal session
+                    set -l val (__config_settings_get_val $name $scope)
+                    test "$val" = DEFAULT; and continue
+                    printf '%s\x1f%s\x1f%s\x1f%s\x1e' var $scope $name $val
+                end
+            case 'sponge_*' __fish_sponge_extra_sensitive \
+                '__fish_scrollback_history_*' __fish_user_dots_path \
+                __fish_user_dots_symlink
+                # Value rows: universal-only, list variables space-joined.
+                set -l val (__config_settings_get_raw $name)
+                test "$val" = DEFAULT; and continue
+                printf '%s\x1f%s\x1f%s\x1f%s\x1e' var universal $name $val
+        end
+    end
+
+    # ── Sub-category taxonomy ─────────────────────────────────────────────
+    for cvar in __fish_config_op_aliases __fish_config_op_autoexec \
+        __fish_config_op_overrides __fish_config_op_integrations \
+        __fish_config_op_logging __fish_config_op_greeting
+        for row in (__config_settings_subcats $cvar)
+            set -l f (string split -- \t $row)
+            printf '%s\x1f%s\x1f%s\x1f%s\x1f%s\x1e' sub $cvar $f[1] $f[2] $f[3]
+        end
+    end
+end