docs(man): explain how an overlay is actually applied

The format was documented; the pipeline was not. The commonest surprise when
writing a local calendar is 'my feast does not appear', and it is almost never
a loading failure -- which means the existing docs answered a question nobody
was asking.

Four stages, named: load, merge, resolve, emit. The one that matters is
resolve, and the point it makes explicit is that overlay entries take part in
precedence on EQUAL TERMS. A local feast is not privileged for being local; it
competes under RG 91's table exactly as a universal one does.

Three concrete causes of a missing feast, each of which came up while testing
this branch: outranked by the day it lands on, a date structurally occupied
(an easter+60 feast can never appear, because Corpus Christi is Easter+60 and
is I class), or a commemoration past RG 111's limit for the day's class. And
the diagnostic rule -- if check says the file loaded and every directive found
its target, the answer is stage 3, so raise the class or move the date.

Also records why the shipped adjustments are applied first and why that is not
configurable: replacing rather than layering would silently drop the
inseparable Peter/Paul commemoration, the Major Litanies, St Barbara and
Rogation Wednesday. Overriding one deliberately by naming its slug is a
different thing from losing it by accident.

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