refactor: papercut cleanup before per-question-type bundle

Two small cleanups that calm the diff for the upcoming §3.A per-question-
type module migration:

1. Rename setScaleLabel → setDateRankedScaleLabel in FormBuilder.svelte.
   The function only handles date_ranked_choice's nested scale.{min,max}_label
   (early-returns on every other type); the standalone scale question type's
   labels use inline handlers. The original name was a half-grown helper from
   the m/fdbck#1 add. The rename removes a real "wait, why does this skip
   scale questions?" papercut that the audit doc flagged.

2. results.ts: replace the (s as ScaleStats & { _sum?: number })._sum
   inline-cast-and-coalesce pattern with proper ScaleStatsWip /
   DateRankedOptionStatsWip accumulator types. _sum is now a real, typed
   field during ingest; finalise reads it and drops it before returning the
   public stats shape. No behaviour change, no new tests needed — existing
   results.test.ts still passes.

   Also: kept `void q;` but expanded the comment to explain that q is the
   schema definition (kept in signature for the upcoming per-type module
   refactor that will dispatch via q.type).

54 tests pass, svelte-check clean, build clean.

src/lib/components/FormBuilder.svelte

@@ -175,7 +175,7 @@
 		update(idx, { options: q.options.filter((_, i) => i !== optIdx) } as Partial<FeedbackQuestion>);
 	}
 
-	function setScaleLabel(idx: number, which: 'min_label' | 'max_label', val: string): void {
+	function setDateRankedScaleLabel(idx: number, which: 'min_label' | 'max_label', val: string): void {
 		const q = value.questions[idx];
 		if (q.type !== 'date_ranked_choice') return;
 		const scale = { ...(q.scale ?? {}) };
@@ -390,7 +390,7 @@
 								maxlength="50"
 								placeholder="e.g. doesn't work"
 								value={q.scale?.min_label ?? ''}
-								oninput={(e) => setScaleLabel(i, 'min_label', (e.target as HTMLInputElement).value)}
+								oninput={(e) => setDateRankedScaleLabel(i, 'min_label', (e.target as HTMLInputElement).value)}
 							/>
 						</div>
 						<div class="fb-question">
@@ -400,7 +400,7 @@
 								maxlength="50"
 								placeholder="e.g. works great"
 								value={q.scale?.max_label ?? ''}
-								oninput={(e) => setScaleLabel(i, 'max_label', (e.target as HTMLInputElement).value)}
+								oninput={(e) => setDateRankedScaleLabel(i, 'max_label', (e.target as HTMLInputElement).value)}
 							/>
 						</div>
 					</div>

src/lib/server/results.ts

@@ -102,10 +102,17 @@ export function aggregateResults(
 	return { total_submissions: subs.length, questions };
 }
 
+// Aggregation-time accumulator types. The `_sum` field is a real, typed
+// member during ingest; finalise reads it and drops it before returning the
+// public stats shape. Cleaner than the previous (s as X & { _sum?: number })
+// inline-cast-and-coalesce pattern.
+type ScaleStatsWip = ScaleStats & { _sum: number };
+type DateRankedOptionStatsWip = DateRankedOptionStats & { _sum: number };
+
 function emptyStats(q: FeedbackQuestion): QuestionStats {
 	switch (q.type) {
-		case 'scale':
-			return {
+		case 'scale': {
+			const wip: ScaleStatsWip = {
 				type: 'scale',
 				count: 0,
 				min: q.min,
@@ -115,7 +122,10 @@ function emptyStats(q: FeedbackQuestion): QuestionStats {
 					value: q.min + i,
 					count: 0,
 				})),
+				_sum: 0,
 			};
+			return wip;
+		}
 		case 'single_choice':
 		case 'multi_choice':
 			return {
@@ -133,15 +143,19 @@ function emptyStats(q: FeedbackQuestion): QuestionStats {
 			return {
 				type: 'date_ranked_choice',
 				count: 0,
-				options: q.options.map((opt) => ({
-					id: opt.id,
-					start: opt.start,
-					end: opt.end ?? null,
-					label: opt.label ?? null,
-					count: 0,
-					mean: null,
-					histogram: [1, 2, 3, 4, 5].map((value) => ({ value, count: 0 })),
-				})),
+				options: q.options.map((opt) => {
+					const wip: DateRankedOptionStatsWip = {
+						id: opt.id,
+						start: opt.start,
+						end: opt.end ?? null,
+						label: opt.label ?? null,
+						count: 0,
+						mean: null,
+						histogram: [1, 2, 3, 4, 5].map((value) => ({ value, count: 0 })),
+						_sum: 0,
+					};
+					return wip;
+				}),
 			};
 	}
 }
@@ -156,11 +170,11 @@ function ingest(
 	switch (s.type) {
 		case 'scale': {
 			if (typeof v !== 'number' || !Number.isFinite(v)) return;
-			s.count++;
-			const bucket = s.histogram.find((b) => b.value === v);
+			const acc = s as ScaleStatsWip;
+			acc.count++;
+			const bucket = acc.histogram.find((b) => b.value === v);
 			if (bucket) bucket.count++;
-			(s as ScaleStats & { _sum?: number })._sum =
-				((s as ScaleStats & { _sum?: number })._sum ?? 0) + v;
+			acc._sum += v;
 			return;
 		}
 		case 'single_choice': {
@@ -205,32 +219,34 @@ function ingest(
 				const raw = ratings[opt.id];
 				if (raw === undefined || raw === null) continue;
 				if (typeof raw !== 'number' || !Number.isInteger(raw) || raw < 1 || raw > 5) continue;
-				opt.count++;
-				const bucket = opt.histogram.find((b) => b.value === raw);
+				const acc = opt as DateRankedOptionStatsWip;
+				acc.count++;
+				const bucket = acc.histogram.find((b) => b.value === raw);
 				if (bucket) bucket.count++;
-				(opt as DateRankedOptionStats & { _sum?: number })._sum =
-					((opt as DateRankedOptionStats & { _sum?: number })._sum ?? 0) + raw;
+				acc._sum += raw;
 				touched = true;
 			}
 			if (touched) s.count++;
 			return;
 		}
 	}
-	void q; // unused after switch covers all branches
+	void q; // q is the schema definition; ingest dispatches on s.type which
+	        // is derived from it via emptyStats. Kept in the signature so a
+	        // future per-type module can take the question for richer logic.
 }
 
 function finalise(s: QuestionStats): void {
 	if (s.type === 'scale') {
-		const sum = (s as ScaleStats & { _sum?: number })._sum;
-		s.mean = s.count > 0 && typeof sum === 'number' ? sum / s.count : null;
-		delete (s as ScaleStats & { _sum?: number })._sum;
+		const acc = s as ScaleStatsWip;
+		s.mean = s.count > 0 ? acc._sum / s.count : null;
+		delete (s as Partial<ScaleStatsWip>)._sum;
 		return;
 	}
 	if (s.type === 'date_ranked_choice') {
 		for (const opt of s.options) {
-			const sum = (opt as DateRankedOptionStats & { _sum?: number })._sum;
-			opt.mean = opt.count > 0 && typeof sum === 'number' ? sum / opt.count : null;
-			delete (opt as DateRankedOptionStats & { _sum?: number })._sum;
+			const acc = opt as DateRankedOptionStatsWip;
+			opt.mean = opt.count > 0 ? acc._sum / opt.count : null;
+			delete (opt as Partial<DateRankedOptionStatsWip>)._sum;
 		}
 		// Sort by mean desc with tiebreaks: count of "5"s, then "4"s, then count desc, then id.
 		s.options.sort((a, b) => {