fix(itn/en): read spoken fractions instead of misfiring the ordinal tagger (#82)

[Alex-Wengg]

Fixes #82.

Problem

nemo_normalize_sentence("use one third of a cup") returned use 4th of a cup.

English ITN had no fraction tagger (fr/de/es/hi/ja/zh do), so one third fell through to the ordinal tagger. Its compound-ordinal path blindly adds prefix + ordinal — correct for twenty first (20 + 1 = 21st) but nonsense for one third (1 + 3 = 4th). The bug hits every ones-cardinal + ones-ordinal denominator (one fifth → 6th, one eighth → 9th, ...); other phrasings leaked the numerator instead (two thirds → 2 thirds, one half → 1 half).

Fix

• New itn::en::fraction tagger, run before ordinal in both the single-expression and sentence pipelines (priority 78, below decimal/above ordinal). Emits N/D:
  • one third → 1/3, one half → 1/2, one quarter → 1/4
  • two thirds → 2/3, three quarters → 3/4, twenty two thirds → 22/3
• Plural/singular disambiguation (the crux): a plural denominator (thirds) is always a fraction; a singular one (third) is a fraction only with numerator one. So twenty third stays the ordinal 23rd, while one third becomes 1/3.
  • Excludes first/second — one second is a duration, not 1/2.
  • Singular scale words stay ordinals: one hundredth → 100th (only two hundredths → 2/100).
  • The article a is not a numerator, so a quarter of the pizza is left unchanged (matches existing behavior/test).
• Ordinal guard: the compound-addition path now requires the prefix to be a round multiple of the next power of ten, so it no longer fabricates ordinals from one + ones-ordinal (one second no longer becomes 3rd). Genuine compounds (twenty first, one hundred tenth) are unaffected.

Tests

• Unit tests in fraction.rs (singular/plural, deferral to ordinals, excluded denominators, article-is-not-a-numerator).
• Integration tests in en_tests.rs for the reported case + ordinal regression guards.
• Full suite green (13 binaries, 0 failures); fst-engine feature still builds; cargo fmt --check clean.

Scope

Default (hand-written Rust) taggers only — the fst-engine path is untouched.