refactor(ssr): Phase 9 — migrate New/Edit Vendor onto the engine (5-step wizard)

A five-step linear wizard (info → terms → account → address → legal) plus a
separate Merge dialog, migrated off mm/* + form-cursor + the EDN snapshot onto the
session-backed engine (wizard2), following the Phase 8 template.

Latent bug found + fixed: the old "Next" PUT /admin/vendor/navigat carried a
[:map [:db/id entity-id]] route-schema on a route with no :db/id path param, so empty
route-params {} → main-transformer's parse-empty-as-nil → nil → 500 on every advance
(the same quirk as Phase 8's query-params, now via route-params). The engine's submit
is a POST with no such schema; the dead navigate route is deleted.

What changed:
- defrecord 5 → 0 (InfoModal/TermsModal/AccountModal/AddressModal/LegalEntityModal +
  VendorWizard), mm/ 0, fc/ cursor refs 0 (wizard AND the de-cursored Merge dialog),
  step-params[…] 0.
- 5 de-cursored step renders (plain data + path->name2 + a *errors* binding); the 3
  repeated grids became add-row-handler + a blank-row row render; the timeline is
  preserved as a per-step side panel.
- :init-fn branches new (empty) vs edit (entity split across the 5 steps' :init-data,
  seeded as per-step step-data so edit opens populated); per-step :validate via
  mc/validate + me/humanize replaces wrap-ensure-step; vendor-step wraps
  handle-step-submit in try+ to surface create-time validation as a 4xx.

Two new gotchas found + fixed + documented:
- empty-step decode: an all-blank step collapses to nil (parse-empty-as-nil), which a
  schema :validate rejects as "invalid type"; decode-with coerces nil → {} so optional-
  only steps advance while required-field steps still fail on the missing key.
- blank nested entity: an untouched Address (all-nil, no :db/id) makes :upsert-entity
  mint a tempid used only as value (datomic error); blank-address? drops it.

Verification: full e2e suite 65/65 (61 prior + 4 new: info renders + timeline; create
across all 5 steps persists; edit opens prefilled and a rename persists; a too-short
name blocks advancing). Create + edit confirmed at the REPL incl. the cookie-session
EDN round-trip. Skill fed (scorecard Phase 9; gotchas for both new traps).

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

src/cljc/auto_ap/routes/admin/vendors.cljc

@@ -9,7 +9,6 @@
              "/account-override" ::new-account-override
              "/account-typeahead" ::account-typeahead
              "/validate" ::validate
-             "/navigat" ::navigate
              "/new" {:get ::new}
              "/merge" {:get ::merge
                        :put ::merge-submit}