feat(text): say so when a text parses into nothing (#278)

[HugoFara]

The backstop for #278, independent of #281.

The silent failure

A language whose Word Characters setting doesn't match the script of its texts does not fail. It parses successfully into nothing:

text saved as TxID 27 — stored items:
   (none)
sentences stored: 1

TokenPersistence::save() has if (empty($tokens)) return; and returns without a word. The text saves, opens, and renders every character — and refuses to respond to any click. No exception, no log line, no message. From the outside it is indistinguishable from an ordinary text that has gone inert, which is exactly what the reporter met, and why their issue says "I don't actually know what the expected behavior is".

One rule, three surfaces

ParseCoverage is the single place that decides a parse came out empty, so the reading view, the check-text page and the API agree on when to speak up.

The test is a word-to-character ratio, not a plain zero. A non-Latin text on a Latin language usually matches a few stray tokens — a digit, a Latin fragment — and a zero test lets those through while the text is still unreadable. The floor is one word per fifty characters, well under the ~1-per-6 of Latin prose or the ~1-per-1 of a character-split script, and it only applies to texts long enough to judge (short titles and captions are exempt).

Surfaces, all ones the reader already reaches:

where	why
Reading view	where the confusion actually happens — a parseWarning on the payload that already feeds it
Check-text page	its whole purpose is "did my parsing work?"; it answered a failed parse with an empty list
ParseText::execute()	a warning field for anything reading the API

Each says what happened and links straight to /languages/{id}/edit.

Verified against a real database

Chinese on Latin defaults   ParseText.warning=no_words   reader banner: SHOWN
Chinese configured right    ParseText.warning=ok         reader banner: none
ordinary English            ParseText.warning=ok         reader banner: none

The banner reads:

None of this text could be turned into words. The language's Word Characters setting does not match this text, so nothing in it can be clicked or learned.
[Check the language settings]

Notes for review

• The banner is built through the DOM rather than by concatenation: text_renderer.ts's escapeHtml() uses textContent/innerHTML, which does not escape quotes. That's fine for text nodes but would let an attribute value break out of its attribute, so href goes through setAttribute. A test covers it.
• While tracing the reading path I found TextReadingService::mainWordLoop() is dead code — no callers. Its query chains select()/selectRaw(), which overwrite rather than accumulate, so it selects only the last five columns and reads six undefined keys. Not touched here; worth a separate look, along with whether the method should exist at all.
• Four new locale keys across all nine languages.

Checks

Psalm 0 errors · PHPCS 0 errors, 0 warnings · PHPUnit 9109 pass (11 new) · Vitest 4330 pass (4 new) · tsc and ESLint clean · assets rebuilt.

One existing frontend test mocked text_renderer exhaustively and needed the new export added to its mock.