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.

bin/pos-ai-alias

@@ -349,8 +349,8 @@ _alias_create() {
     local name="$preset_name"
     while true; do
         if [ -z "$name" ]; then
-            step 1 4 "Alias Name" >&2
-            name="$(menu_ask_value "Alias name" "")" || return 0
+            step 1 5 "Alias Name" >&2
+            name="$(menu_ask_value --allow-empty "Alias name" "")" || return 0
         fi
         [ -z "$name" ] && { warn "Alias name cannot be empty" >&2; name=""; continue; }
         if ! _alias_name_valid "$name"; then
@@ -380,7 +380,7 @@ _alias_create() {
     done
 
     # Step 2: Provider
-    step 2 4 "Provider" >&2
+    step 2 5 "Provider" >&2
     local pidx
     pidx="$(_alias_provider_pick)" || return 0
     local providers=()
@@ -407,7 +407,7 @@ _alias_create() {
     local prompt=""
     while true; do
         step 4 5 "System Prompt" >&2
-        prompt="$(menu_ask_value "System prompt (empty = use built-in)" "")" || return 0
+        prompt="$(menu_ask_value --allow-empty "System prompt (empty = use built-in)" "")" || return 0
         if [[ "$prompt" == *'|'* ]]; then
             warn "System prompt must not contain '|' characters" >&2
             prompt=""; continue