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, 30 insertions, 0 deletions

diff --git a/mobile/mobile_bench_test.go b/mobile/mobile_bench_test.go
new file mode 100644
index 0000000..7e95744
--- /dev/null
+++ b/mobile/mobile_bench_test.go
@@ -0,0 +1,30 @@
+package mobile
+
+import "testing"
+
+// BenchmarkDaysWeek (mobile_test.go) already covers the OF 7-day view this
+// perf fix targets -- see internal/readings/offline.go's offlineLoad, which
+// currently re-stacks the calendar layers and re-parses the book table once
+// per day inside the loop Days drives. The benchmarks below add the two
+// comparisons that let that fix's win be measured: the same view in the
+// traditional form (a different, larger embedded calendar layer), and a
+// single Day call, the per-day unit of work Days repeats.
+
+// BenchmarkDays7EF is the 7-day view in the traditional (1962) form, whose
+// calendar layer is a different embedded file (Tridentine vs Universal).
+func BenchmarkDays7EF(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		Days("2026-07-27", 7, "ef", "pl")
+	}
+}
+
+// BenchmarkDay1 measures a single Day call, the unit of work Days repeats.
+// Comparing this to BenchmarkDaysWeek/7 shows how much of each day's cost is
+// the date-independent setup this fix hoists out.
+func BenchmarkDay1(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		Day("2026-07-27", "of", "vul", "pl")
+	}
+}