feat(log): QSO number column, oldest contact = 1

Not the id: the primary key follows insertion order, so importing an old ADIF
gives the oldest contacts the highest ids. This is a rank over qso_date.

Computed over the WHOLE log, not the query result — ranking inside the result
would renumber every contact the moment a filter is applied, and QSO #1 would
change identity as the operator typed.

Held as one id-to-rank map, built from a single ordered id query and dropped
with the other derived indexes when the log changes. A contact logged from the
entry form is the newest, so it takes the next number without rereading the
log: AddQSO deliberately avoids full invalidation because a contest run would
pay for it once per QSO. A contact entered with an OLDER date belongs in the
middle of the order, so there the map is dropped and rebuilt rather than
mis-numbered.

qso_number_test.go

@@ -0,0 +1,95 @@
+package main
+
+import (
+	"testing"
+	"time"
+
+	"hamlog/internal/qso"
+)
+
+func at(s string) time.Time {
+	t, err := time.Parse("2006-01-02 15:04", s)
+	if err != nil {
+		panic(err)
+	}
+	return t
+}
+
+// The number is the contact's rank in the WHOLE log, not its position in what
+// the grid happens to be showing. Ranking inside the result would renumber
+// every contact the moment a filter is applied, and QSO #1 would change
+// identity as the operator typed.
+func TestQSONumberIsGlobalNotPerPage(t *testing.T) {
+	a := &App{}
+	// The log, oldest first: ids are deliberately NOT in date order, which is
+	// exactly what an imported ADIF produces.
+	a.qsoNumbers = map[int64]int{
+		770: 1, // oldest, imported last so it has the highest id
+		101: 2,
+		102: 3,
+		103: 4,
+	}
+
+	// A filtered page holding only two of them, newest first as the grid asks.
+	page := []qso.QSO{{ID: 103}, {ID: 770}}
+	a.stampQSONumbers(page)
+
+	if page[0].Number != 4 {
+		t.Errorf("id 103 numbered %d, want 4", page[0].Number)
+	}
+	if page[1].Number != 1 {
+		t.Errorf("id 770 numbered %d, want 1 — the oldest contact, whatever its id", page[1].Number)
+	}
+}
+
+// A contact logged now is the newest, so it takes the next number without
+// rereading the log — a full scan per QSO would be felt in a contest run.
+func TestNewQSOTakesTheNextNumber(t *testing.T) {
+	a := &App{}
+	a.qsoNumbers = map[int64]int{1: 1, 2: 2, 3: 3}
+	a.qsoNumMax = at("2026-08-13 10:00")
+
+	a.noteQSONumbered(9, at("2026-08-13 11:00"))
+	if got := a.qsoNumbers[9]; got != 4 {
+		t.Errorf("new QSO numbered %d, want 4", got)
+	}
+	if !a.qsoNumMax.Equal(at("2026-08-13 11:00")) {
+		t.Error("the newest date was not carried forward")
+	}
+}
+
+// A contact entered with an OLDER date belongs in the middle of the order.
+// Appending it would number it last, which is wrong — so the map is dropped and
+// rebuilt correctly instead.
+func TestBackdatedQSOForcesARebuild(t *testing.T) {
+	a := &App{}
+	a.qsoNumbers = map[int64]int{1: 1, 2: 2, 3: 3}
+	a.qsoNumMax = at("2026-08-13 10:00")
+
+	a.noteQSONumbered(9, at("2020-01-01 09:00"))
+	if a.qsoNumbers != nil {
+		t.Errorf("a back-dated QSO was appended as the newest: %v", a.qsoNumbers)
+	}
+}
+
+// Nothing built yet: the lazy build will see the new contact anyway, so this
+// must not create a one-entry map that then numbers the whole log wrongly.
+func TestNoteBeforeAnyBuildDoesNothing(t *testing.T) {
+	a := &App{}
+	a.noteQSONumbered(9, at("2026-08-13 11:00"))
+	if a.qsoNumbers != nil {
+		t.Errorf("built a map from a single contact: %v", a.qsoNumbers)
+	}
+}
+
+// With no index available the column is simply empty — never wrong.
+func TestStampWithoutIndexLeavesZero(t *testing.T) {
+	a := &App{} // no qso repo, so the index cannot be built
+	page := []qso.QSO{{ID: 1}, {ID: 2}}
+	a.stampQSONumbers(page)
+	for _, q := range page {
+		if q.Number != 0 {
+			t.Errorf("invented a number without an index: %d", q.Number)
+		}
+	}
+}