Add the Greek forefeast labels and Greek-dated commemorations

[brianglass]

The second half of the remaining title differences from #228 (the first is #255). The two PRs are independent and can merge in either order.

Added for Greek

Date	Greek	antiochian.org
Jun 25	Leavetaking of the Nativity of the Forerunner	2 of 2 years
Jul 23	Hieromartyr Phocas, Bishop of Sinope	2 of 2
Oct 28	Protection of the Most-Holy Theotokos	4 of 5
Dec 8	Forefeast of the Conception	8 of 8
Dec 21-23	Forefeast of Nativity	8 of 8 each

How

• Labels: these are feast names, which an override row can't set (Day.OVERRIDABLE). Each common row (612, 737, 778, 791-793) becomes slavic, with an identical greek copy (950-955) that adds the name. Oct 1's Protection already has this shape.
• Phocas: a new greek commemoration on Jul 23, linked to his existing Saint. He keeps Sep 22 in both traditions, as antiochian.org lists him on both dates.
• Fixtures: spliced by hand rather than re-dumped, to avoid reordering about 22k lines.

Unchanged

• Fasting, rank and saints on these dates.
• Slavic output.
• The fasting golden file and the goarch.org fasting audit.

Deliberately left alone

• Dec 26: we show the Synaxis of the Theotokos, which antiochian.org also uses as the headline in most years.
• Jan 8-13 and Feb 3: antiochian.org headlines these as afterfeasts, but goarch.org headlines the day's saints. A feast name would outrank those saints in summary_title and move us away from goarch.org.

Both are documented in docs/greek-commons.md, pass 13.

Tests

New test_greek_only_feast_labels and test_phocas_on_july_23_for_greek. The full suite passes locally except the known MonthCacheTestCase failure (local DummyCache).

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