docs(skill): record faked-cursor de-fake learning + Phase 2 scorecard progress

- gotchas.md: de-faking a cursor is not a drop-in -- with-field-default mutates the
  cursor (transact!) as a render side effect and broke the simple-mode swap; the de-fake
  belongs with the render-fn rewrite, verified against the swap spec.
- scorecard.md: append the Phase 2 (in-progress) Transaction Edit row -- no-cursor 1->0,
  LOC 1608->1555, parity held (swap 6/6 + Shared Location). Faked roots / snapshot /
  Selmer / route-collapse remain as the wholesale-rendering continuation of Phase 2.

.claude/skills/ssr-form-migration/reference/gotchas.md

@@ -83,6 +83,19 @@ validation re-render; a `#error {…}` stack means a 500. Then serialize the for
 before save (`new FormData(document.querySelector('#wizard-form'))`) to see exactly what
 posts. This is how the `:mode` 500 and the empty-account bugs above were isolated.
 
+## De-faking a cursor is not a drop-in — `with-field-default` mutates
+
+Tempting fix for a faked deep cursor (`with-cursor` + synthetic `MapCursor` at index 0):
+replace it with `(fc/with-field-default 0 {})` to advance naturally. **It broke the
+simple-mode swap** (`transaction-edit-swap` test 1 threw). `with-field-default` calls
+`cursor/transact!` — it *mutates the form cursor* (assoc-ing the default row) as a render
+side effect, which changes simple-mode behavior. The read-only synthetic `MapCursor` did
+not. Lesson: removing a faked cursor on these modals is **not** a one-liner — it's part of
+the larger render-fn extraction (render the row from explicit data, construct field names
+directly, look up errors explicitly), done when the simple/advanced rows are reworked into
+pure render fns / Selmer. Don't swap one cursor primitive for another and assume parity;
+verify against the swap spec, and expect the de-fake to come with the render-fn rewrite.
+
 ## Scorecard exceptions (ratchet violations with a reason)
 
 _None yet._ Append here if a migration must let a metric regress for a documented reason.