Announce whole-document replacements so spell check can re-run

[Wavesonics]

setText replaces the document without emitting an edit operation, so nothing downstream can tell its text is gone. Spell check learned about a document only when the platform checker became available, which on Android beats the markdown import often enough that the one-shot check runs over an empty document and no squiggles ever appear.

The sample app worked around this with a second LaunchedEffect(spellChecker, markdownExtension) that re-ran the full check itself. Any host loading content programmatically needed the same workaround, and HtmlExtension's import had none.

TextEditorState now exposes documentReplacements, emitted from replaceContent (whose only callers are the two setText overloads), and SpellCheckingTextEditor runs a full check on it. The sample app's workaround is deleted.

Either race order costs one real scan: whichever of the checker and the document arrives last triggers the check, and the earlier one finds either no words or no checker. That removes the duplicate concurrent full check at load without needing any serialization.

Notes

BasicTextEditor calls setText when its text parameter changes, so a host driving text from state now gets a full re-check on each change. That is the intended behavior, but it is a behavior change rather than a pure cleanup.

A separate signal rather than a synthetic edit operation: emitting a real operation would be arguably more correct but ripples into undo/redo and every existing editOperations collector.

This is independent of #94, which stays drafted.

Tests

DocumentReplacementSignalTest covers the signal itself (setText emits, an edit does not). Two cases in SpellCheckE2eTest cover the behavior through the real editor stack: a document loaded into a composed editor gets checked, and replacing a document checks the new text. Both fail with the collector reverted; verified.

[codacy-production]

Up to standards ✅

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