kernel(temporal): Holy Thursday is white, per RG128(b)'s named exception

RG 128(b) excepts "Missa sive Chrismatis sive in Cena Domini feria V
Hebdomadae sanctae" from the Septuagesima-to-Easter-Vigil violet span
as a whole-Mass exception (unlike Palm Sunday's blessing/procession,
which the same sentence carves out as only part of that day) -- so
Holy Thursday's Mass is white, not violet. temporal_ef.ml fell through
to the generic Passiontide ferial path (season_colour Passiontide =
violet) for this date, since [named] has no entry for the Triduum at
all (rank was already correct via [privileged_feria]; only colour was
wrong).

Found via Task 16's missalemeum oracle comparison: every other Triduum
day's oracle colour set includes violet as an option (Good Friday
"bv", Holy Saturday "vw" -- already-flagged, deliberately deferred
per-action gaps, register §3b), but Holy Thursday's is white alone.

This is a genuine NEW divergence from lectio too (lectio has no such
exception and still prints violet for Holy Thursday) -- added as
Layer C11 to the differential harness and data/ef/expected-divergences
.sexp, 46 rows (2005-2050), rather than silently changing what that
harness accepts.

Also corrected a stale comment on season_colour claiming white's RG
paragraph was never pinned -- RG 119 was found and cited in the
register on 2026-08-11 but the correction was never copied into this
file's own comment.

1 files changed, 28 insertions, 5 deletions

diff --git a/lib/rites/rite_ef/temporal_ef.ml b/lib/rites/rite_ef/temporal_ef.ml
index 52a9adb..b39da79 100644
--- a/lib/rites/rite_ef/temporal_ef.ml
+++ b/lib/rites/rite_ef/temporal_ef.ml
@@ -314,13 +314,15 @@ let privileged_feria d =
   (n >= -6 && n <= -1) || (n >= 1 && n <= 6) || (n >= 50 && n <= 55)
 
 (* RG 117 enumerates the five colours (white, red, green, violet, black);
-   RG 127 assigns green and RG 128 violet to the seasons de Tempore below.
-   White's own specific paragraph (the "B) De colore albo" section, between
-   117 and 123) was not pinned by the primary-source search available here --
-   left uncited rather than guessed; see register §3 "Colours". *)
+   RG 127 assigns green and RG 128 violet to the seasons de Tempore below. RG
+   119 (register §3b, primary-source-verified 2026-08-11 -- this comment was
+   stale until Task 16 noticed the correction had not been copied down here):
+   white "a festo Nativitatis Domini usque ad expletum tempus Epiphaniae"
+   and "a Missa Vigiliae paschalis usque ad Missam vigiliae Pentecostis
+   exclusive" -- exactly Christmastide and Paschaltide below. *)
 let season_colour = function
   | Advent | Septuagesima | Lent | Passiontide -> Colour.Violet (* RG 128 *)
-  | Christmastide | Paschaltide -> Colour.White
+  | Christmastide | Paschaltide -> Colour.White (* RG 119 *)
   | Time_after_epiphany | Time_after_pentecost -> Colour.Green (* RG 127 *)
 
 (* Gaudete (Advent III) and Laetare (Lent IV) are rose: RG 131, "may be used...
@@ -411,6 +413,27 @@ let temporal d =
                       let colour =
                         (* The Pentecost octave weekdays are red, not Paschaltide's white. *)
                         if days_between easter d >= 50 && days_between easter d <= 55 then Colour.Red
+                          (* RG 128(b) (docs/research/rules-register.md §3b), primary
+                             text: "...a dominica in Septuagesima usque ad Vigiliam
+                             paschalem, EXCEPTIS: ... MISSA SIVE CHRISMATIS SIVE IN
+                             CENA DOMINI FERIA V HEBDOMADAE SANCTAE; ..." -- violet
+                             runs Septuagesima to the Easter Vigil EXCEPT (among
+                             others) "the Mass, whether of the Chrism or in Cena
+                             Domini [Holy Thursday], on Thursday of Holy Week" --
+                             named as a WHOLE-MASS exception (unlike Palm Sunday's
+                             "blessing and procession of palms", which the SAME
+                             sentence carves out as only PART of that day, register
+                             §3b's own RG126 note on the not-yet-modelled per-action
+                             nuance), so this is a clean whole-day colour fact, not
+                             a per-action one the day/colour model cannot express.
+                             Task 16, found via the missalemeum oracle comparison:
+                             every other Triduum day's oracle colour SET includes
+                             violet as one option (Good Friday "bv", Holy Saturday
+                             "vw" -- RG 132's black is a separate, ALREADY-flagged
+                             gap, register §3b, not touched here), but Holy
+                             Thursday's is white ALONE -- confirming this specific
+                             day, and only this one, needs the exception coded. *)
+                        else if days_between easter d = -3 then Colour.White
                         else season_colour s
                       in
                       let week_n = week d in