Greek and Slavic usage, and the marks the books print

Both keep the same Typikon and read it a little differently, and until now
this library silently picked one reading and called it the Typikon. Two
differences are modelled, each one visible in the output and named in the
day's reason:

  - The Protection of the Theotokos is a great feast in Slavic usage. Greek
    practice ranks it lower, and the Church of Greece keeps it on 28 October
    besides. This has teeth: rank is what lifts a Wednesday or Friday fast.
  - Greek practice admits fish on any day of the Nativity Fast but Wednesday
    and Friday until 17 December; Slavic practice keeps fish to Saturdays and
    Sundays throughout.

Every jurisdiction carries the usage its books follow, and --tradition
overrides it for a parish that does not match its jurisdiction's default. The
difference in the calendar lives in a data overlay compiled in beside the
common menologion, which is where a local church's own commemorations would
go; the difference in the fast lives in the rule, because that is where it is.

Each rank also now carries the mark the books print beside it, circled cross,
cross and Jerusalem cross for great, vigil and polyeleos. Unicode has no glyph
for the polyeleos sign, a cross over a half-circle, so the Jerusalem cross
stands in as digital calendars generally settle on.

src/rules/orthodox.rs

@@ -24,6 +24,8 @@
 //!   addition, which this vocabulary has no level for.
 //! - **Great Week** is kept at xerophagy on its weekdays whatever saint falls
 //!   in it, because the days of the Passion outrank the menologion.
+//! - **Fish in the Nativity Fast** follows the usage, Greek or Slavic. See
+//!   `Tradition`.
 //! - **Monastic practice**, and the Holy Mountain in particular, is stricter
 //!   than any of this throughout.
 //!
@@ -32,6 +34,7 @@
 use super::{Context, Rule, Vocabulary};
 use crate::calendars::Weekday;
 use crate::fasting::{Fast, Level};
+use crate::jurisdictions::Tradition;
 use crate::menologion::Rank;
 use crate::seasons::Season;
 
@@ -183,6 +186,16 @@ pub fn fast_for(ctx: &Context) -> Fast {
             }
             return for_feast(Level::Xerophagy, ctx, Level::WineOil, "the forefeast of the Nativity");
         }
+        // Greek practice permits fish on any day but Wednesday and Friday
+        // until 17 December; Slavic practice keeps it to Saturdays and
+        // Sundays. This is the one place the two usages part on the fast.
+        if ctx.tradition == Tradition::Greek && md <= (12, 17) {
+            if matches!(ctx.weekday, Weekday::Wednesday | Weekday::Friday) {
+                return for_feast(Level::Xerophagy, ctx, Level::Fish,
+                                 "Wednesday and Friday of the Nativity Fast");
+            }
+            return Fast::customary(Level::Fish, "the Nativity Fast, Greek usage");
+        }
         if weekend {
             return Fast::customary(Level::Fish, "Saturday and Sunday of the Nativity Fast");
         }