feat(config): add sub-category cascade evaluator

functions/__fish_config_op_cascade.fish

@@ -0,0 +1,63 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   __fish_config_op_cascade <category_variable> [<subcategory_variable>]
+#
+# DESCRIPTION
+#   Evaluates the opt-out cascade for one C1-C6 classification: an
+#   explicit truthy/falsy sub-category variable wins outright; otherwise
+#   falls back to the category variable; otherwise falls back to the
+#   master switch __fish_config_opinionated. A category in the opt-in
+#   list (currently just __fish_config_op_logging, C5) defaults to
+#   disabled when nothing in the chain is explicit, and the master switch
+#   cannot enable it -- this is data-driven here instead of a hardcoded
+#   string comparison, so any sub-category nested under an opt-in
+#   category inherits "off unless explicit" for free through the cascade,
+#   with no per-sub-category special-casing.
+#
+# ARGUMENTS
+#   category_variable     Name (without $) of a C1-C6 category variable
+#   subcategory_variable  Optional name (without $) of a sub-category
+#                          variable nested under that category
+#
+# EXIT STATUS
+#   0  Enabled
+#   1  Disabled
+#
+# EXAMPLE
+#   __fish_config_op_cascade __fish_config_op_aliases
+#   __fish_config_op_cascade __fish_config_op_aliases __fish_config_op_aliases_filesystem
+function __fish_config_op_cascade --description 'Evaluate the sub-category -> category -> master opt-out cascade'
+    set -l opt_in_categories __fish_config_op_logging
+
+    set -l chain $argv[1]
+    if test (count $argv) -ge 2 -a -n "$argv[2]"
+        set chain $argv[2] $argv[1]
+    end
+
+    for var_name in $chain
+        __fish_variable_check $var_name
+        set -l s $status
+        if test $s -eq 0
+            return 0
+        end
+        if test $s -eq 1
+            return 1
+        end
+    end
+
+    # Every variable in the chain was unset/unrecognized. chain[-1] is
+    # always the category variable (present whether or not a
+    # sub-category was given) -- opt-in categories default to disabled
+    # and the master switch cannot override that.
+    if contains -- $chain[-1] $opt_in_categories
+        return 1
+    end
+
+    __fish_variable_check __fish_config_opinionated
+    if test $status -eq 1
+        return 1
+    end
+    return 0
+end