[visualfc]

Opening a file can freeze LiteIDE when syntax highlighting rules repeatedly match without consuming text. The hang report shows the main thread inside Highlighter::iterateThroughRules during rehighlighting after a file is opened.

Bound rule processing without character progress to 1024 checks per offset, and limit recursive child/fallthrough evaluation to 128 levels. On either limit, use the existing broken-highlighter fallback to stop syntax highlighting for the document while preserving its text. Clear the context stack on failure and log the block, offset, and context.

Add a standalone QtTest target covering nine scenarios: look-ahead with #stay, base-context #pop, context cycles, empty string matches, fallthrough loops, recursive children, consuming rules, and valid look-ahead transitions. Normal cases also verify formatting of a 4096-character line.

Validation:

• Original code reproduced the base-context look-ahead #pop hang and required a test timeout after 5 seconds.
• Qt 5.15.19 and Qt 6.11.1: all nine scenarios passed (11 QtTest results including setup/cleanup).
• Qt 6 text editor library and liteeditor plugin builds passed.
• git diff --check passed.

The report does not identify the exact triggering file or rule; the regression test reproduces the same class of non-progressing rule loop.