feat(#351): implement daily-only CFFDRS detection — GREEN

Follows b32ccfd (RED) to green. Implements the two pure functions whose
contracts and tests were committed there.

hasDailyOnlyCffdrs — true only when some rows carry codes and others do
not. All-codes is a conforming firestarr_csv; no-codes is a different
failure with nothing to recover.

findStartingCodeCandidate — the latest row that carries all three codes,
sits at local hour 12 or 13 in the supplied zone, and falls strictly
before ignition. Minutes are ignored; stations poll at :00, :05, :06 or
:10 and all are the same daily reading.

Corrected one test. 'excludes a daily reading exactly at ignition' passed
the full row set, where Aug 1 and Aug 2 also carry readings strictly
before the boundary — so the correct answer was 76.3, not null, and no
rule could return null there without breaking two other tests. Narrowed
to a single row so it isolates the boundary it names.

Invalid-timezone handling returns null rather than failing fast. Filed as
353, not fixed here — untested behaviour does not belong mid-cycle.

15 tests green. Backend suite 646 passed (was 631). tsc --noEmit exit 0.

Author: spydmobile

backend/src/infrastructure/weather/__tests__/dailyCffdrs.test.ts

@@ -136,8 +136,12 @@ describe('findStartingCodeCandidate', () => {
   });
 
   it('excludes a daily reading exactly at ignition — strictly before', () => {
+    // A single row, so nothing earlier can be returned instead. This isolates
+    // the boundary: the reading is at a daily hour and carries codes, and is
+    // rejected solely for being at ignition rather than before it.
     const atIgnition = new Date('2026-08-03T19:06:00Z');
-    expect(findStartingCodeCandidate(vitasRows(), atIgnition, ZONE)).toBeNull();
+    const rows = [daily('2026-08-03T19:06:00Z', 84.65, 20.72, 424.9)];
+    expect(findStartingCodeCandidate(rows, atIgnition, ZONE)).toBeNull();
   });
 
   it('returns null when no daily reading precedes ignition', () => {

backend/src/infrastructure/weather/dailyCffdrs.ts

@@ -64,8 +64,14 @@ function hasCodes(row: CffdrsRow): boolean {
  * Returns false when every row has codes (a conforming firestarr_csv) and false
  * when no row has codes (nothing to recover — a different failure).
  */
-export function hasDailyOnlyCffdrs(_rows: CffdrsRow[]): boolean {
-  throw new Error('not implemented');
+export function hasDailyOnlyCffdrs(rows: CffdrsRow[]): boolean {
+  if (rows.length === 0) return false;
+
+  const withCodes = rows.filter(hasCodes).length;
+
+  // Both sides must be non-empty. All-codes is a conforming firestarr_csv;
+  // no-codes is a different failure with nothing to recover.
+  return withCodes > 0 && withCodes < rows.length;
 }
 
 /**
@@ -79,14 +85,34 @@ export function hasDailyOnlyCffdrs(_rows: CffdrsRow[]): boolean {
  * Returns null when no such reading exists — the caller must not invent one.
  */
 export function findStartingCodeCandidate(
-  _rows: CffdrsRow[],
-  _ignition: Date,
-  _timezone: string,
+  rows: CffdrsRow[],
+  ignition: Date,
+  timezone: string,
 ): StartingCodeCandidate | null {
-  throw new Error('not implemented');
-}
+  let best: CffdrsRow | null = null;
+
+  for (const row of rows) {
+    if (!hasCodes(row)) continue;
+    if (row.datetime.getTime() >= ignition.getTime()) continue;
+
+    const local = DateTime.fromJSDate(row.datetime, { zone: timezone });
+    if (!DAILY_READING_LOCAL_HOURS.includes(local.hour)) continue;
 
-// Referenced by the implementation; declared above to keep the contract visible.
-void DAILY_READING_LOCAL_HOURS;
-void hasCodes;
-void DateTime;
+    if (best === null || row.datetime.getTime() > best.datetime.getTime()) {
+      best = row;
+    }
+  }
+
+  if (best === null) return null;
+
+  const local = DateTime.fromJSDate(best.datetime, { zone: timezone });
+
+  return {
+    ffmc: best.ffmc,
+    dmc: best.dmc,
+    dc: best.dc,
+    observedAt: best.datetime,
+    // Minutes are deliberately dropped — the reading names an hour, not an instant.
+    localLabel: `${local.toFormat('yyyy-MM-dd, HH')}00`,
+  };
+}