test(differential): allow-list C12 for the 15-entry rank correction

Re-bootstrapping sanctoral.sexp (previous commit) makes 7 of the 15
newly-corrected III-class feasts win an occurrence they previously lost as
Commemoration_only: RG 91 entry 24 (a universal III-class feast) outranks
entry 28 (a IV-class feria), and the occurrence code that resolves this
already existed and needed no change. lectio's own committed differential
fixture (test/fixtures/lectio-ef-2005-2050.txt) is a frozen snapshot
predating lectio's own equivalent generator fix, so it still shows the
class-4 feria observed with the same saint only as a trailing
commemoration -- a new, expected divergence from that fixture, 14 rows in
the 2005-2050 window.

expected_rows (14) is independently re-derived by grepping the regenerated
sanctoral.sexp's own post-regeneration civil-day output against the
committed lectio fixture directly (two flat-file extractions cross-checked
against each other), not transcribed from the comparator's own failing-test
output.

C12 reuses an id last used, and removed, by the RG16(a) task's own reverted
Purification retag -- noted at both the old and new entries' own comments
so the id's history is visible to a future reader.

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