fix: pos ai alias create aborts on empty system prompt — menu_ask_value --allow-empty

User report: pressing Enter on 'System prompt (empty = use built-in)'
silently returned to the menu — no alias created, and step labels read
[1/4] [2/4] in a 5-step flow.

Detective (pre-existing, not a 2026-09-06 regression): menu_ask_value's
documented contract is 'rc 1 = cancel, or empty answer with no default';
the step-4 call passed an empty default so the advertised empty answer
hit rc 1 and '|| return 0' aborted the flow. Same latent trap at the
alias-name step (empty-name warn/re-prompt was dead code). 11 other
call sites are correct (6 external rely on empty=cancel, 4 pass
defaults) — no global semantic change allowed.

Architect: opt-in --allow-empty flag on menu_ask_value (backward
compatible; empty+no-default -> rc 0 + empty value; genuine cancel/EOF
stays rc 1; default still wins). Builder: implemented in lib/menu-lib.sh
+ bin/pos-ai-alias (steps 1-2 relabeled /5, flag at the two approved
sites); 7-case smoke matrix PASS.

Tests: tests/t-menu-allow-empty.sh (30 checks) — semantics matrix
against the real menu_ask_value via non-TTY stdin, reader-contract
probes (empty-Enter rc 0 vs EOF rc 1), static guards on step labeling,
the exactly-2 flag call sites, edit-flow untouched, and a scope fence
over all pos-* tools. Pty E2E proven feasible (script -qec, 3 scenarios)
and documented in the Tester report; the E2E file itself remains a
follow-up.

Verified: make gen idempotent; make check OK; make lint 0 FAIL, 0 WARN;
make test 17 files / 299 checks / 0 fail / 0 skip (~49s); bash -n clean.

lib/menu-lib.sh

@@ -24,7 +24,7 @@
 #   menu_guard                          rc 0 iff stdin is a terminal
 #   menu_run <title> <item...>          numbered menu loop → chosen index
 #   menu_pick <prompt> <item...>        type-to-filter picker → chosen index
-#   menu_ask_value <label> [default]    prompted value → entered text
+#   menu_ask_value [--allow-empty] <label> [default]    prompted value → entered text
 #   menu_read_value <label>             raw-mode bracketed-paste reader
 #   menu_redraw                         internal redraw (menu_read_value only)
 
@@ -347,15 +347,23 @@ menu_redraw() {
 # text — including multi-line pastes — inserts it literally instead of letting
 # leftover lines escape to the shell as commands.
 #   rc 0  value on stdout · rc 1  EOF/cancel, or empty answer with no default.
+#   With --allow-empty: empty answer with no default → rc 0 + empty value;
+#   only genuine cancel/EOF returns rc 1.
 menu_ask_value() {
+    local allow_empty=0
+    if [ "${1:-}" = "--allow-empty" ]; then
+        allow_empty=1
+        shift
+    fi
     local label="$1" def="${2:-}" val pr="$1"
     [ -n "$def" ] && pr="$pr [$def]"
     if ! val="$(menu_read_value "$pr")"; then
         return 1                          # EOF / cancel
     fi
     if [ -z "$val" ]; then
-        [ -n "$def" ] || return 1
-        echo "$def"
+        [ -n "$def" ] && { echo "$def"; return 0; }
+        [ "$allow_empty" -eq 1 ] || return 1
+        echo ""
         return 0
     fi
     echo "$val"