maybe_ascii: fold š/č/ş/ș/ț/ĉ and Cyrillic щ/ц to digraphs

[pudo]

Fixes #292.

maybe_ascii folded š/č/ş to bare s/c, but name spellings in the wild mostly use sh/ch. The corpus measurement in #292 shows a net gain of +5,214 agreeing spellings for the six Latin characters.

Change

• Latin digraph fold: š/ş/ș → sh, č/ĉ → ch, ț → ts. This runs after the per-script pass and before the NFKD mark strip, which otherwise removes the diacritic. The input is NFC-composed first, so decomposed input (s + U+030C) is covered too.
• Cyrillic pre-pass: щ → shch, ц → ts, substituted before the ICU4X Cyrillic transliterator. The transliterator's output for these (ŝ, c) can't be told apart from other letters afterwards.
• ž and ţ keep folding to the bare letter: for ž the corpus is a coin flip, and for ţ the current fold is better.
• Uppercase maps to title case (Š → Sh), following the existing Þ → Th.

input	before	after
Kašerininov	Kaserininov	Kasherininov
Шевченко	Sevcenko	Shevchenko
Хрущёв	Hrusev	Hrushchev
Цой	Coj	Tsoj
Şükür	Sukur	Shukur

Metaphone runs on the ASCII form, so its codes now agree with the common spellings (Kašerininov and Kasherininov both give KXRNNF).

Downstream impact

This changes the output of maybe_ascii, and therefore NamePart.ascii, comparable and metaphone, plus address tokens (шоссе → shosse, which forms.yml already lists). Anything that stores folded forms, such as yente indexes, should be rebuilt after upgrading. This needs a changelog note and a version bump.

Not covered here: х → h (vs kh) and й → j (vs i/y).

Tests

• cargo test: 273 passed
• pytest: 548 passed
• mypy --strict: clean
• clippy (with and without --features python): clean
• cargo fmt --check: clean

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