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.

README.md

@@ -67,6 +67,22 @@ which agrees with the Gregorian for every date until 2800. So a new-calendar
 church keeps the Nativity with the West and Pascha with the East, not a
 compromise but exactly what the 1923 congress decided.
 
+### Greek or Slavic usage
+
+Both keep the same Typikon and read it a little differently. Two of those
+differences are modelled, and each is visible in the output:
+
+| | Slavic | Greek |
+|---|---|---|
+| The Protection, 1 October | great feast | lower rank; Greece keeps it on 28 October besides |
+| Fish in the Nativity Fast | Saturdays and Sundays | any day but Wednesday and Friday, until 17 December |
+
+Every jurisdiction carries the usage its books follow. `--tradition slavic` or
+`--tradition greek` overrides it, which is what you want for a parish that does
+not match its jurisdiction's default.
+
+### Which calendar?
+
 Pick a reckoning without naming a church:
 
 ```console
@@ -137,6 +153,10 @@ $ typikon --format ics --days 365 > orthodox.ics
 $ typikon --year 2027                    # a year's key dates, to check against yours
 ```
 
+Every day of vigil rank or above carries the mark its books print beside it:
+⊕ for a great feast, ✚ for a vigil, ☩ for a polyeleos. It is the `sign`
+field in the JSON and a column in the delimited formats.
+
 `--no-emoji` drops the glyphs from every format, for a terminal or a bar
 without them. `--list-fields` prints the columns `csv` and `tsv` can take, and
 `--fields` picks a subset in the order you name them. `--json` and `--ics`