ja: handle furigana — two kinds, opposite treatment (closes #704) - #705

Before: a parenthesised reading was phonemized as ordinary text after
the kanji it annotates, so the reading came out TWICE —
`漢字（かんじ）` read *känd͡ʑi känd͡ʑi* where a reader says it once.

The issue proposed one rule. Implementing it turned up a distinction
worth splitting on: TRUE RUBY POSITIONING IS NOT PLAIN TEXT, so what
reaches a phonemizer is one of several flattenings of markup — and they
do not all mean the same thing.

  DECLARED — Unicode interlinear annotation (U+FFF9 base U+FFFA ruby
  U+FFFB, defined for exactly this and discouraged for interchange, so
  rare) and the aozora `｜base《ruby》`. These SAY they are ruby, so the
  author has stated the reading: the RUBY WINS and the base is dropped.
  `｜日本《にっぽん》` now reads にっぽん, overriding the default にほん —
  which is the entire reason for writing it.

  PARENTHESISED — `base（ruby）` / `base(ruby)`. A CONVENTION, not a
  declaration; the same shape is an ordinary parenthetical. So the guard
  is EQUALITY with the computed reading, and a mismatch means "keep
  both" rather than "the author is overriding".

The three cases the issue named as must-not-change all survive, because
none equals the computed reading:
  日本（にほんじん）    a gloss
  日本（にっぽん）      an alternate reading in the AMBIGUOUS form
  会議（ミーティング）  a katakana gloss of a loanword

Note 日本（にっぽん） and ｜日本《にっぽん》 now differ, and that is the
point: the parenthesised one is a parenthetical the author may have
meant as a contrast, the declared one is an instruction.

The test asserts the property that actually matters — an annotated
sentence is BYTE-IDENTICAL to the same sentence written without the
annotation, including at the token boundary the rewrite creates. That
caught the one thing worth checking: dropping the annotation joins 東京
directly to に, and the result must equal `東京に行く`. It does.

3114 → 3118 tests; typecheck clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Lc3WnUgogC7okV7n53vjr