Recover spoken "year" number forms in the reverse-table backend

[jqueguiner]

Problem

The generic reverse-table backend (every locale except en) matches only
num2words' single canonical spelling of a value. Speech, however, routinely
uses the year reading, which is never the canonical form — so words2num
raised cannot parse on:

lang	input	expected
de	neunzehn neunundneunzig	1999
nl	negentien zevenennegentig	1997
nl	negentienhonderdzevenennegentig	1997
sv	nittonhundranittiosju	1997
fr	dix-neuf cent quatre-vingt-dix	1990
it	mille novecento ottantotto	1988

For a speech-facing library, years are among the most common spoken numbers.

Fix (all in words2num2-core, table-driven, no per-language grammar)

• hundred_word(lang) — derives the language's hundred morpheme
  (cent / cento / hundert / honderd / hundra) by rendering
  num2words(200..=900) and taking the longest common suffix. Probing the full
  range makes the pre-hundred vowel vary (it quattr-o / se-i / nov-e), so
  the morpheme itself is isolated rather than a unit's trailing vowel.
• parse_year(lang, text) — recovers L*100 + R from an explicit hundred
  token (fr), a hundred glued into one token (sv/nl), or two 2-digit groups
  (de/nl), resolving every part through the existing reverse-table primitive.
• additive hundreds in parse_scaled_inner — "novecento" (900) + 88,
  covering the vowel elision num2words applies when gluing
  (novecent-ottantotto), which a naive de-space cannot reproduce.
• de-spaced retry in base_convert for glued canonicals.

Every path is last-resort: it runs only after the whole-string table hit and
parse_scaled both decline, so a canonical number (vingt trois = 23) is
matched first and never reaches the fallback. No canonical round-trip changes.

Tests

• New rust tests: hundred_morpheme_is_derived, spoken_year_forms,
  year_forms_via_public_entry (end-to-end through words2num).
• Full rust suite green; the 133 existing python tests still pass.
• End-to-end wheel check: the 6 cases above now resolve, and canonical /
  scaled / connector round-trips (deux mille dix, soixante-neuf mille huit,
  dos mil veinticuatro, …) are unchanged — 19/19.