feat: add __fish_palette shared output palette helper

Declared --no-scope-shadowing with a bare set, so the 12 colour roles land
in the caller's scope under the same short names the consuming functions
already interpolate. Keeping the names means the conversion never edits an
output string in 33 of the 38 consumers.

No consumer is converted yet; byte-identity harness still reports 29/29.

tests/functional.fish

@@ -97,6 +97,33 @@ function test_vault_dir_honors_override
     test "$got" = /tmp/vault-override-check
 end
 
+function test_palette_roles_defined
+    functions -q __fish_palette
+    or begin
+        echo "    __fish_palette is not defined"
+        return 1
+    end
+    # Called from inside a function, the palette must land in THIS scope.
+    __fish_palette
+    set -l missing
+    for role in c_reset c_head c_cmd c_arg c_flag c_warn c_err c_ok \
+        c_accent c_dim c_sel c_hi
+        if not set -q $role; or test -z "$$role"
+            set -a missing $role
+        end
+    end
+    if test (count $missing) -gt 0
+        echo "    palette roles empty or unset: $missing"
+        return 1
+    end
+    # Nothing may leak to global scope.
+    if set -q -g c_reset
+        echo "    __fish_palette leaked c_reset into global scope"
+        return 1
+    end
+    return 0
+end
+
 function functional_test_main
     set -l names (functions -a | string match 'test_*' | sort)
     set -l failed 0