caldata: allow an external universal sanctorale to override the embedded one

The universal calendar of saints can now be refreshed or replaced without a
rebuild: if ~/.config/lectio/sanctorale/of.ini (OF) or ef.ini (EF) is present
and parses, caldata.Base uses it instead of the embedded calendar; a missing or
malformed file falls back to the embedded default, so the binary stays
self-contained and correct out of the box. Local diocesan/national additions
keep layering on top via calendars/ + `use` (unchanged). The temporal cycle and
precedence rules stay in code -- only the sanctorale is data.

- caldata.SetSanctoraleDir hook (mirrors bible/naming/i18n); Base() prefers the
  external file per form. config.SanctoraleDir(); wired in the CLI, TUI and web
  entry points. Tests cover override, no-file, and malformed-file fallback.
- README: new "Calendar of saints" section (two tiers: overlay via `use`,
  replace via sanctorale/<form>.ini); config comment notes the override.

1 files changed, 47 insertions, 0 deletions

diff --git a/internal/caldata/sanctorale_test.go b/internal/caldata/sanctorale_test.go
new file mode 100644
index 0000000..610aa33
--- /dev/null
+++ b/internal/caldata/sanctorale_test.go
@@ -0,0 +1,47 @@
+package caldata
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestBaseSanctoraleOverride(t *testing.T) {
+	dir := t.TempDir()
+	os.WriteFile(filepath.Join(dir, "of.ini"), []byte(
+		"[layer]\nid = test-override\ntype = universal\n\n"+
+			"[test-saint]\ndate = 06-15\nrank = feast\nname.en = Test Saint\n"), 0o644)
+	SetSanctoraleDir(dir)
+	t.Cleanup(func() { SetSanctoraleDir("") })
+
+	// External file present and valid -> it REPLACES the embedded OF calendar.
+	l := Base("new")
+	if l.ID != "test-override" {
+		t.Fatalf("external of.ini not used: layer id = %q", l.ID)
+	}
+	if _, ok := l.Cels["test-saint"]; !ok || len(l.Cels) != 1 {
+		t.Fatalf("external calendar not loaded verbatim: %d cels", len(l.Cels))
+	}
+
+	// EF form has no external file here -> embedded Tridentine still used.
+	if ef := Base("old"); ef.ID == "test-override" {
+		t.Fatal("EF wrongly took the OF override")
+	}
+}
+
+func TestBaseFallsBackToEmbedded(t *testing.T) {
+	// No dir set -> embedded default.
+	SetSanctoraleDir("")
+	if l := Base("new"); l.ID != "universal" || len(l.Cels) < 100 {
+		t.Fatalf("no-override should give the embedded calendar, got id=%q n=%d", l.ID, len(l.Cels))
+	}
+
+	// A malformed external file must not break computation -> embedded fallback.
+	dir := t.TempDir()
+	os.WriteFile(filepath.Join(dir, "of.ini"), []byte("[unclosed section\n"), 0o644)
+	SetSanctoraleDir(dir)
+	t.Cleanup(func() { SetSanctoraleDir("") })
+	if l := Base("new"); l.ID != "universal" || len(l.Cels) < 100 {
+		t.Fatalf("malformed override should fall back to embedded, got id=%q n=%d", l.ID, len(l.Cels))
+	}
+}