test(config-settings): cover the empty state dump refusal

The guard added in 208ad95 had no test. This is the failure worth
covering, because it is the one that does not announce itself: with an
empty dump every row renders as DEFAULT, indistinguishable from a config
where nothing is set, so the user toggles from a false baseline.

Reaching the guard needs a real terminal -- the isatty check sits in
front of it -- so the case runs fish under a pty via python3's stdlib
pty module, which this suite already depends on. An empty
__config_settings_state is shadowed in to fake the failure.

The pty reader's 15s deadline is load-bearing rather than defensive: if
the guard regresses, config-settings does not fail, it opens the TUI and
blocks on getch(), so an unbounded read would hang the suite instead of
failing it. Verified both ways -- passes with the guard, and with the
guard removed the deadline fires and the case fails with a legible
excerpt rather than a screenful of escape sequences.

tests/test-config-settings-tui.fish

@@ -53,4 +53,75 @@ function test_no_tty_is_refused_cleanly
 end
 check "no TTY is refused with a message, not a curses crash" true (test_no_tty_is_refused_cleanly; and echo true; or echo false)
 
+# An empty state dump is the dangerous failure, because it is not one: the TUI
+# would render every row as DEFAULT, which is indistinguishable from a config
+# where nothing is set. The user would be looking at ON rows reported as OFF's
+# neighbour and toggling from a false baseline. The launcher must refuse.
+#
+# Reaching that guard needs a real terminal -- the isatty check sits in front
+# of it -- so this runs fish under a pty. python3 is already a hard
+# prerequisite of this suite (see the header), so its stdlib pty module costs
+# no new dependency; `script` would.
+#
+# The deadline is load-bearing, not belt-and-braces. If the guard regresses,
+# config-settings does not fail -- it opens the TUI and blocks on getch(),
+# so an unbounded read here would hang the suite instead of failing it.
+set -l pty_runner '
+import os, pty, select, signal, sys, time
+pid, fd = pty.fork()
+if pid == 0:
+    os.execvp("fish", ["fish", "--no-config", "-c", sys.argv[1]])
+deadline, out = time.monotonic() + 15, b""
+while time.monotonic() < deadline:
+    if not select.select([fd], [], [], deadline - time.monotonic())[0]:
+        break
+    try:
+        chunk = os.read(fd, 65536)
+    except OSError:
+        break
+    if not chunk:
+        break
+    out += chunk
+else:
+    out += b"\nTIMEOUT: the TUI opened and blocked on input\n"
+try:
+    os.kill(pid, signal.SIGKILL)
+except ProcessLookupError:
+    pass
+os.waitpid(pid, 0)
+sys.stdout.write(out.decode("utf-8", "replace"))
+'
+
+function test_empty_state_dump_is_refused --argument-names runner
+    # Shadowing the autoloaded __config_settings_state with an empty function
+    # is what fakes the failure; a real dump always carries the taxonomy.
+    set -l cmd "set -p fish_function_path $repo_root/functions;
+        function __config_settings_state; end;
+        config-settings;
+        echo RC=\$status"
+    set -l out (python3 -c "$runner" "$cmd" | string collect)
+
+    set -l failed 0
+    if not string match -q '*produced no output*' -- $out
+        # A regression here means the TUI drew itself, so $out is a screenful
+        # of escape sequences. Strip them and keep a usable excerpt.
+        set -l seen (string replace -ra '\e\[[0-9;?]*[a-zA-Z]|\e[()][A-Z]' '' -- $out \
+            | string join ' ' | string sub -l 120)
+        echo "    expected the empty-dump refusal, got: $seen"
+        set failed 1
+    end
+    if not string match -q '*RC=1*' -- $out
+        echo "    expected exit status 1 from the refusal"
+        set failed 1
+    end
+    # Guard the guard: if the pty were not a terminal we would be watching the
+    # isatty check fire and would learn nothing about the dump.
+    if string match -q '*needs a terminal*' -- $out
+        echo "    the isatty guard fired -- the pty did not present a terminal"
+        set failed 1
+    end
+    test $failed -eq 0
+end
+check "an empty state dump is refused, not rendered as all-DEFAULT" true (test_empty_state_dump_is_refused "$pty_runner"; and echo true; or echo false)
+
 report