The reorder event snapshot could previously show the reverse pass's
stale timeout label. Set a sentinel before each click so the
ProgramChanged handler can only observe the label applied by the
current operation's highlighting pass.
Quality review of 50884da found the unshown RichTextBox oracle compared
cached CRLF text against the highlighted editor's native LF-normalized
text, which would fail on first Windows execution. Compare generated
text with normalized line endings on both sides instead, capture the
status label and preview state inside the ProgramChanged handler to
prove fallback-before-notify ordering, document the HighlightSpan
UTF-16 and rule-index contract, and pin the null-text argument check.
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