Deprecate the MECAB magic word in LgRegexpWordCharacters

[HugoFara]

LgRegexpWordCharacters = 'MECAB' is a magic string: the field that should hold a word-characters regex instead holds a literal marker meaning "tokenize with MeCab". LgParserType now exists to say that properly, so the magic word should go — but it is not a one-line swap, because it carries a second fact that LgParserType does not.

It means two different things

Ten PHP sites read it, and they do not agree on what it is.

As a parser selector — genuinely superseded by LgParserType:

• Language::getEffectiveParserType() (Language.php:434)
• ParserRegistry::resolveParserType() / resolveParserTypeFromRow() (ParserRegistry.php:199,228)
• ExternalParser.php:551

As "this language has no spaces between words" — not superseded, and this is the blocker. In SentenceService it appears in six places alongside LgSplitEachChar and LgRemoveSpaces, governing how spaces are reconstructed around tokens:

// SentenceService.php:333, 362 (and 136, 290, 455, 548)
if (!$removeSpaces && !($splitEachChar || 'MECAB' == strtoupper(trim($termchar)))) {

Setting LgParserType = 'mecab' and clearing the field would silently change Japanese spacing, because nothing else tells SentenceService the language is space-free. That fact needs an explicit home first — LgRemoveSpaces already exists, or it could be derived from the parser's own capabilities.

It also consumes the field it occupies: a MeCab language has no word-characters regex at all, which interacts badly with anything reasoning about word characters (see #278).

It is still a live input path

Not a legacy artifact to read-only-support. The word-characters selector still offers "MeCab (recommended)" (locale/en/language.json → form.regexp_alt_mecab), and language_form.ts::wordCharChange() writes the literal string into the field. New magic-word languages are still being created today.

Meanwhile the modern path has already moved on without it: langdefs.json's Japanese preset declares "parserType": "mecab", and #281 is what makes that preset take effect.

A bug it has already caused

Every reader normalizes before comparing — strtoupper(trim($x)) === 'MECAB' — except one:

// GetPhoneticReading.php:54
if ($wordCharacters !== "mecab") {
    return $text;
}

A language whose field holds MECAB in any other casing (an old install, an import, a hand-edit) silently gets no phonetic reading: no error, just the input back. Worth fixing on its own, independently of this issue.

Why it matters beyond tidiness

Parser identity currently lives in three places — the magic word, LgSplitEachChar, and LgParserType — and 20251223_120000_add_parser_type.sql backfills the third from the first two. That ambiguity is what made #281 change tokenization for every CJK language on an upgraded install: it started trusting a column holding derived values. The fix there (17262f5) reads a type that merely restates its neighbouring flag as the flag rather than as intent — correct for now, and a workaround for this issue. Retiring the magic word is what would let LgParserType simply mean what the user chose, and let that workaround be deleted.

Suggested plan

1. Give "no spaces between words" an explicit home, so SentenceService stops asking about the magic word.
2. Migration: for UPPER(TRIM(LgRegexpWordCharacters)) = 'MECAB', set LgParserType = 'mecab' (the backfill already did), set the space flag, and put a real regex (or empty) back in the word-characters field.
3. Drop the MeCab option from the word-characters selector, so nothing new is written; the parser dropdown already covers it.
4. Keep the readers accepting the magic word for one release, logging a deprecation.
5. Next release: delete the readers, and with them the restatesALegacySignal() workaround in ParserRegistry.

Roughly 10 PHP sites, 2 TS files, 1 migration, 1 locale key. Small, but it changes how Japanese renders, so it wants a before/after parse comparison across every language — the same one that caught the #281 regression.