v0.9.0 - robustness and hackability

Audit of the shipped program and the fixes it found.

- citation numbers must be plain digits and positive: Luke 1_1:5 used to
  read as chapter ELEVEN via OCaml integer-literal syntax, and a
  descending range parsed happily
- four pairs of different books shared a full title (1/2 Corinthians,
  Thessalonians, Timothy, Peter), leaving 108 citations in 2027 alone
  unresolvable to a book; a test now forbids it, while allowing two ids
  that are the same book under different numbering
- spec 8.5 delivered: shipped styles re-parse their own rendered output,
  down from 32 of 52 Latin abbreviations failing to none beyond
  same-book aliases
- overlay errors humanised at every load path
- new-overlay scaffold shows citations at the right nesting level
- config --show validates before printing; errors no longer echo whole
  file lines
- a month answers to both spellings of its own name

--raw output is byte-identical to v0.8.0 across the whole domain.