refactor(ssr): Phase 7 — migrate Invoice Pay onto the engine; prove the cross-step merge

Invoice Pay is the first GENUINE multi-data-step wizard, and migrating it exercises
the engine's central abstraction for the first time: choose-method collects
{:bank-account :method}, payment-details collects {:invoices :check-number
:handwritten-date :mode}, and the engine's get-all MERGES the two independent step
payloads for the per-method pay (handwrite-check transacts a pending check; the
others go through print-checks-internal). This is exactly the mechanism the Phase-6
adversarial review flagged as unproven.

What changed
- Deleted the 3 wizard records (PayWizard / ChoosePaymentMethodModal /
  PaymentDetailsStep), MultiStepFormState, the EDN snapshot, and the step-params[...]
  prefix. Replaced with pay-wizard-config (init-fn builds read-only :context;
  two steps; done-fn = pay!) driven by wizard2.
- De-cursored the payment-details amounts grid (fc/cursor-map -> explicit
  (map-indexed) over :context :invoices with path->name2 names).
- The bank-account cards' method controls now post {bank-account, method,
  direction:next} straight to the engine submit-route (was a bespoke navigate route).
- Routes 3 -> 2: open-pay-wizard (GET), pay-step (every transition); the
  pay-wizard-navigate route is deleted.
- Used the post-review engine primitives: :open-response (modal wrap), nav-footer
  (with new :save-label "Pay"), auto nav-field stripping (flat decode, no allowlist),
  Enter guard.

invoices.clj falls fully off the framework: Invoice Pay was the last mm/fc user
(bulk-edit went in Phase 5), so fc/ 0, mm/ 0, defrecord 0, step-params 0 — and the
multi-modal / form-cursor / malli.util requires are removed.

Gotcha discovered + documented: wizard session data must be EDN-safe (the cookie
session store has no clj-time readers), so the date default is computed in render,
not stored in context.

Verification: invoice-pay spec 3/3 (the merge end-to-end); full suite 58/58; load-file
clean; cljfmt clean. Skill fed: scorecard row (merge proven; whole-file zeroing) +
the EDN-session-safety gotcha.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/clj/auto_ap/ssr/components/wizard2.clj

@@ -67,9 +67,10 @@
    field the engine branches on; the advance/save button is marked `data-primary` so the
    form's Enter guard triggers it. Also renders the `#form-errors` slot.
 
-     (nav-footer {:next \"Test\"})            ; an intermediate step: Next
-     (nav-footer {:back? true :save? true})  ; the last step: Back + Save"
-  [{:keys [next back? save?]}]
+     (nav-footer {:next \"Test\"})                              ; intermediate step: Next
+     (nav-footer {:back? true :save? true})                    ; last step: Back + Save
+     (nav-footer {:back? true :save? true :save-label \"Pay\"}) ; last step, custom label"
+  [{:keys [next back? save? save-label]}]
   [:div.flex.justify-end.items-baseline.gap-x-4
    [:div#form-errors]
    (when back?
@@ -77,7 +78,7 @@
    (when next
      (com/button {:type "submit" :name "direction" :value "next" :data-primary "" :color :primary :class "w-24"} next))
    (when save?
-     (com/button {:type "submit" :name "direction" :value "submit" :data-primary "" :color :primary :class "w-24"} "Save"))])
+     (com/button {:type "submit" :name "direction" :value "submit" :data-primary "" :color :primary :class "w-24"} (or save-label "Save")))])
 
 (defn blank-row
   "A fresh repeated-row map for an 'add row' interaction, with a temp `:db/id` (so a row