Phase 6b/7b: per-child decoded periods, separate brief blocks, idempotent calendar (AUD-119)

- Parse each child's own "Today" day widget for the timetable (FR-16); drop the
  school-wide week calendar's lesson grid and clean the leaked routine rows.
- Add the editable codes map (FR-17): subjects, rooms, teachers and period
  labels, seeded, an unknown code falls back to the raw code; `school codes` CLI.
- Brief: Finn's block first, then Harry's, four sections always present (FR-9);
  periods decoded into plain English; the undated tail is one counted line (FR-18).
- Calendar: write periods + work due + work overdue + major events, per child,
  on every pass. Identity is the stable per-item key, so a re-run never re-inserts
  (FR-8). morning-brief.sh now runs scan -> calendar push -> notify.
- Day-widget labels are 12-hour: 1-6 read as PM, fixing afternoon periods.

Proven live on .13: image 0.6.2-phase6b7b; both children's briefs decoded and
separate; 35 events in calendar.school; runs 2 and 3 of the scan added nothing.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/store/calendar_test.go

@@ -113,3 +113,36 @@ func TestCalendarPushSummaryIsAppendOnly(t *testing.T) {
 		t.Fatalf("last = %+v", summary.Last)
 	}
 }
+
+// TestCodesSeedAndEdit proves the decode map is seeded, editable and that a
+// correction survives a re-seed (FR-17).
+func TestCodesSeedAndEdit(t *testing.T) {
+	ctx := context.Background()
+	st := newTestStore(t)
+
+	codes, err := st.Codes(ctx)
+	if err != nil {
+		t.Fatalf("codes: %v", err)
+	}
+	if codes.Subject["7SCF"] != "Year 7 Science" {
+		t.Errorf("7SCF = %q, want the seeded Year 7 Science", codes.Subject["7SCF"])
+	}
+	if codes.Subject["NOPE"] != "" {
+		t.Errorf("unknown code should be missing, got %q", codes.Subject["NOPE"])
+	}
+
+	if _, err := st.SetCode(ctx, "teacher", "KNI", "Mr Smith"); err != nil {
+		t.Fatalf("set: %v", err)
+	}
+	again, err := st.Codes(ctx)
+	if err != nil {
+		t.Fatalf("codes again: %v", err)
+	}
+	if again.Teacher["KNI"] != "Mr Smith" {
+		t.Errorf("KNI = %q, want the correction to survive the re-seed", again.Teacher["KNI"])
+	}
+
+	if _, err := st.SetCode(ctx, "bogus", "X", "Y"); err == nil {
+		t.Error("an unknown kind should be rejected")
+	}
+}