merge: make the calendar engine stop repeating whole-year work per day

A seven-day view resolved the year seven times. Each readings.Load stacked
the calendar data layers and re-read the Bible book table -- both
date-independent -- and the EF path rebuilt the year's entire transfer plan
and its occupancy scan on every single day.

Three logic-neutral fixes: hoist the date-independent setup out of the Days
loop; memoise the EF transfer plan on (year, Selection, content hash of the
merged sanctoral); and build the occupancy index once alongside it rather
than scanning per candidate.

EF's seven-day view goes 74ms to 16ms, its multiple over OF from ~13x to
~3x. OF is unchanged, as it never touched the EF paths.

Output is byte-identical throughout, proven by SHA-256 over a 492-case
sweep spanning both forms, both languages, four corpora, the Triduum, a
Requiem day and the three Joseph/Annunciation collision years. The caches
are keyed on content, not identity, so a user overlay invalidates them --
asserted by a test that varies the overlay and requires the answer to
change, itself mutation-proved by dropping the content hash and watching it
fail.

1 files changed, 45 insertions, 0 deletions

diff --git a/internal/readings/readings_test.go b/internal/readings/readings_test.go
index 1674d85..c81e324 100644
--- a/internal/readings/readings_test.go
+++ b/internal/readings/readings_test.go
@@ -1,6 +1,7 @@
 package readings
 
 import (
+	"reflect"
 	"strings"
 	"testing"
 
@@ -136,6 +137,50 @@ func TestSundayRankIsDisplayOnly(t *testing.T) {
 	}
 }
 
+// TestLoadWithAgreesWithLoad guards the fast path multi-date callers (e.g.
+// mobile.Days) use to avoid re-stacking the calendar layers and re-parsing
+// the book table once per date: LoadWith, given a cfg's own Prepared value,
+// must return exactly what Load(cfg, opts) returns, for every date and both
+// forms. This is the correctness backstop for the perf fix -- Prepare only
+// hoists WHEN the date-independent setup happens, never WHAT it computes.
+func TestLoadWithAgreesWithLoad(t *testing.T) {
+	dates := []string{
+		"2026-07-22", // ordinary weekday
+		"2028-02-29", // leap day
+		"2026-04-09", // Holy Thursday 2026
+		"2026-04-10", // Good Friday 2026
+		"2026-04-11", // Holy Saturday 2026
+		"2026-11-02", // All Souls (a Requiem day)
+		"2026-12-25", // Christmas
+	}
+	for _, lect := range []string{"new", "traditional"} {
+		cfg := config.Config{Lectionary: lect}
+		p := Prepare(cfg)
+		for _, date := range dates {
+			opts := Options{Date: date, All: true}
+			wantSecs, wantInfo, wantErr := Load(cfg, opts)
+			gotSecs, gotInfo, gotErr := LoadWith(p, cfg, opts)
+			if (wantErr == nil) != (gotErr == nil) {
+				t.Fatalf("%s %s: Load err=%v, LoadWith err=%v", lect, date, wantErr, gotErr)
+			}
+			if wantErr != nil {
+				continue
+			}
+			if gotInfo != wantInfo {
+				t.Errorf("%s %s: LoadWith info = %+v, want %+v", lect, date, gotInfo, wantInfo)
+			}
+			if len(gotSecs) != len(wantSecs) {
+				t.Fatalf("%s %s: LoadWith %d sections, want %d", lect, date, len(gotSecs), len(wantSecs))
+			}
+			for i := range wantSecs {
+				if !reflect.DeepEqual(gotSecs[i], wantSecs[i]) {
+					t.Errorf("%s %s: section %d = %+v, want %+v", lect, date, i, gotSecs[i], wantSecs[i])
+				}
+			}
+		}
+	}
+}
+
 // TestLoadTraditional computes the Extraordinary Form day offline: it never
 // needs the network, and yields the EF epistle+gospel with a header name.
 func TestLoadTraditional(t *testing.T) {