docs(skill): add cookbook entry for de-faking a fixed-index row from explicit data

Captures the reusable pattern proven on simple-mode-fields*: render a known-index row
(accounts[0]) from explicit data with explicit field names (path->name2 equivalent) +
explicit error lookup, instead of faking a deep cursor. Pairs with the gotcha that
with-field-default mutates the cursor. Growth contract for the de-fake commit.

.claude/skills/ssr-form-migration/reference/component-cookbook.md

@@ -90,6 +90,35 @@ replaces the amount input (caret survives).
                  :placeholder "Optional note"})   ; no hx-* — rides along to save
 ```
 
+## fixed-index row from explicit data — de-faking a deep cursor
+
+When a row always lives at a known index (e.g. simple mode renders exactly `accounts[0]`),
+render it from **explicit data with explicit field names** instead of faking a cursor
+rooted there. Build the name the same way the cursor would (`path->name2`) and read errors
+from the same path — no `with-cursor`/`MapCursor` rebind, no `with-field-default` (which
+*mutates* the cursor and breaks swap behavior, see `gotchas.md`).
+
+```clojure
+(defn- account-field-name [index field]            ; == path->name2 for this path
+  (str "step-params[transaction/accounts][" index "]["
+       (if (keyword? field)
+         (str (when (namespace field) (str (namespace field) "/")) (name field))
+         field) "]"))
+
+(defn- account-field-errors [index field]
+  (when (bound? #'fc/*form-errors*)
+    (get-in fc/*form-errors* [:step-params :transaction/accounts index field])))
+
+;; render the row directly -- no fc/with-field / fc/with-cursor wrappers
+[:span
+ (com/hidden {:name (account-field-name 0 :db/id) :value row-id})
+ (com/validated-field {:errors (account-field-errors 0 :transaction-account/account)}
+   (account-typeahead* {:name (account-field-name 0 :transaction-account/account) ...}))
+ ...]
+```
+Verify byte-parity against the cursor version (the swap spec's simple-mode tests catch
+divergence). Scorecard heuristic 1: faked roots → 0.
+
 ## mode toggle ($/% radio, simple/advanced link) — Rule 3, whole-form swap
 
 ```clojure