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/build.rs

@@ -196,6 +196,13 @@ pub struct PlanNeed {
     /// start and not after, where they depart instead.
     #[serde(default)]
     pub turned_down_from: Vec<String>,
+    /// What the person must already have or be for this alternative to
+    /// work: a membership, a reference number, the link from a mail,
+    /// an account. Stated on the card before the form, because a
+    /// condition a reader meets only after filling something in has
+    /// already cost them the filling in. Empty when there is none.
+    #[serde(default)]
+    pub requires: String,
     /// Who presses the button that moves a case INTO a stage, where it
     /// is not `handled_by`: another group, several groups, or
     /// `submitter` - the person who sent the form, through their own
@@ -628,10 +635,14 @@ pub fn fallback_words(plan: &Plan) -> BTreeMap<String, Value> {
             "button": "Send",
             "encouragement": "",
             "icon": "",
-            // Two sections of context the plan already knows: the road
-            // a case travels, and who is at the other end. The
-            // fallbacks are built from the plan, so they are true
-            // before a model has said a word.
+            // Three sections of context the plan already knows: what
+            // the person must have for this to work, the road a case
+            // travels, and who is at the other end. The fallbacks are
+            // built from the plan, so they are true before a model has
+            // said a word.
+            "requires_title": if need.requires.trim().is_empty() { String::new() } else { "What you need".to_string() },
+            "requires_description": need.requires.trim(),
+            "requires_icon": if need.requires.trim().is_empty() { "" } else { "key" },
             "road_title": "What happens next",
             "road_description": stages_sentence(need),
             "road_icon": "route",
@@ -1190,6 +1201,17 @@ fn form_alternative(plan: &Plan, need: &PlanNeed, w: &Value, action: &str) -> Va
         f
     };
     let mut features = Vec::new();
+    // What the person must already have or be for this to work at all,
+    // first of everything: a card that advertises something conditional
+    // and says so only after the form has taken their time has already
+    // wasted it.
+    // Whether the section exists is the plan's to say, never the
+    // wording's: under a lift every slot holds a token, and a token is
+    // never empty, so a word-driven condition would conjure a section
+    // the live page does not have and shift every feature after it.
+    if !need.requires.trim().is_empty() {
+        features.push(strip(s(&w["requires_title"]), s(&w["requires_description"]), s(&w["requires_icon"])));
+    }
     if !s(&w["road_description"]).trim().is_empty() {
         features.push(strip(s(&w["road_title"]), s(&w["road_description"]), s(&w["road_icon"])));
     }
@@ -1718,7 +1740,7 @@ mod tests {
             lands_in: "b".into(), handled_by: "g".into(),
             stages: v(stages), done_when: v(done), success: v(success), skippable: v(skippable),
             moves_by: BTreeMap::new(), watched_by: vec![],
-            events: BTreeMap::new(), attended_by: None, mails: BTreeMap::new(), stages_text: BTreeMap::new(), turned_down_from: vec![],
+            events: BTreeMap::new(), attended_by: None, mails: BTreeMap::new(), stages_text: BTreeMap::new(), turned_down_from: vec![], requires: String::new(),
             collects: vec![], max_fields: 3,
         }
     }
@@ -1797,6 +1819,24 @@ mod tests {
         assert!(!m.contains_key("won"));
     }
 
+    #[test]
+    fn a_stated_prerequisite_comes_before_the_form() {
+        let yaml = "business: B\ncontact: { name: I, email: i@b.example }\nneeds:\n  - id: renew\n    who: members\n    wants: another year\n    requires: You need your membership number from last year's receipt.\n    lands_in: renewals\n    handled_by: office\n    stages: [open, done]\n    done_when: [done]\n    max_fields: 2\n    collects:\n      - { name: number }\n";
+        let plan: Plan = serde_yaml::from_str(yaml).unwrap();
+        plan.validate().unwrap();
+        let files = render(&plan, &fallback_words(&plan)).unwrap();
+        let front: Value = serde_yaml::from_str(&files["questions/index.yaml"]).unwrap();
+        let features = front["alternatives"][1]["features"].as_array().unwrap();
+        assert_eq!(features[0]["name"], "What you need");
+        assert_eq!(features[0]["description"], "You need your membership number from last year's receipt.");
+        assert_eq!(features[1]["name"], "What happens next");
+        // No prerequisite, no section.
+        let none: Plan = serde_yaml::from_str(&yaml.replace("    requires: You need your membership number from last year's receipt.\n", "")).unwrap();
+        let files = render(&none, &fallback_words(&none)).unwrap();
+        let front: Value = serde_yaml::from_str(&files["questions/index.yaml"]).unwrap();
+        assert_eq!(front["alternatives"][1]["features"][0]["name"], "What happens next");
+    }
+
     #[test]
     fn a_case_is_turned_down_only_where_the_plan_says() {
         let mut n = need(&["open", "invited", "active", "departed", "declined"], &["departed", "declined"], &["departed"], &[]);