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, 26 insertions, 1 deletions

diff --git a/internal/caldata/caldata.go b/internal/caldata/caldata.go
index 9c17035..4e54f1f 100644
--- a/internal/caldata/caldata.go
+++ b/internal/caldata/caldata.go
@@ -77,9 +77,34 @@ func Tridentine() calendar.Layer {
 	return l
 }
 
+// sanctoraleDir, when set, holds an optional external universal sanctorale that
+// overrides the embedded one (of.ini for OF, ef.ini for EF). Set once at startup
+// via SetSanctoraleDir; empty means "use the embedded calendar".
+var sanctoraleDir string
+
+// SetSanctoraleDir points caldata at the dir holding an optional external
+// universal sanctorale (<dir>/of.ini, <dir>/ef.ini). Empty disables it.
+func SetSanctoraleDir(dir string) { sanctoraleDir = dir }
+
 // Base returns the universal base layer for a form: the 1962 calendar for
-// "old" (EF), the General Roman Calendar otherwise (OF).
+// "old" (EF), the General Roman Calendar otherwise (OF). If an external
+// sanctorale file is present and parses, it REPLACES the embedded calendar for
+// that form -- so the saints and ranks can be updated without a rebuild. A
+// missing or malformed external file falls back to the embedded default, which
+// therefore always keeps the tool self-contained and correct out of the box.
 func Base(form string) calendar.Layer {
+	if sanctoraleDir != "" {
+		name := "of.ini"
+		if form == "old" {
+			name = "ef.ini"
+		}
+		if data, err := os.ReadFile(filepath.Join(sanctoraleDir, name)); err == nil {
+			if l, err := ParseLayer(data); err == nil {
+				return l
+			}
+			// malformed external file: fall through to the embedded default
+		}
+	}
 	if form == "old" {
 		return Tridentine()
 	}