build: a card states what the reader must already have

`requires:` on a need is one sentence naming what the person must
have or be for the alternative to work - a membership, a reference
number, the link from a mail. It renders as the card's first feature,
above the road, who-reads-it and the form, because a condition a
reader meets only after filling the form in has already cost them the
filling in. Whether the section exists is read from the plan, never
from its wording: under a lift every slot holds a token and a token is
never empty, which would conjure a section the live page lacks and
shift every feature after it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/plan.rs

@@ -70,6 +70,10 @@ needs:
     done_when: [<end>, <end>] # every stage that counts as finished, good or bad
     success: [<end>]          # the finished stages that are a good outcome for the business
     skippable: [<stage>]      # stages a case may pass over (papers already complete)
+    requires: <one sentence>    # what the person must ALREADY have or be for this to work:
+                              # a membership, a reference number, the link from a mail, an
+                              # account. Stated on the card above the form. Omit when there
+                              # is none; never invent one
     turned_down_from: [open, <stage>]
                               # the unfinished stages a case can still be turned down from;
                               # omit for every one. An applicant is declined before they
@@ -122,6 +126,11 @@ Rules:
 - Every need whose form collects an email should tell the person on
   arrival and on the decisions about them (`mails`); the plan then
   needs `mail: { from }`.
+- Every feature on a card explains itself: a list says what one row is
+  and what its buttons do, a form says what to have ready. If an
+  alternative only works for someone who already has something, say so
+  in `requires` - a condition the reader meets only after filling the
+  form in has already cost them the filling in.
 - Needs that land in the same bucket must have the same stages,
   done_when, skippable, moves_by, watched_by and handled_by.
 - Every required field is one the business will use to act on the