Extract rule: template-require-iframe-title

[NullVoxPopuli]

Split from #2371.

[NullVoxPopuli-ai-agent]

Review: template-require-iframe-title

General Correctness

1. Duplicate title detection differs from original. The original reports BOTH the original iframe (with a "This title is not unique" message) and the duplicate iframe (with a more detailed message including the duplicate value). The PR only reports the second (duplicate) iframe, not the first. This means the first iframe with a duplicated title never gets flagged, which diverges from the original behavior. The original also includes numbered references (#1, #2) in its messages.

2. Error messages simplified. The original has distinct messages for duplicates ("This title is not unique. #N" for the first, and a detailed message for the duplicate). The PR uses the same missingTitle messageId for all cases. This loses context -- the user won't know if the issue is a missing title, empty title, or duplicate title. Consider adding separate messageIds like duplicateTitle.

3. The knownTitles array is instance-level state in the create() closure. This works for a single file, but note that the original uses this.knownTitles = [] in the visitor() method, resetting per-file. The PR's approach of declaring knownTitles in the create() function achieves the same per-file scoping since create() is called per file. Correct.

4. aria-hidden value check. The original checks for the presence of the aria-hidden attribute (any value). The PR does the same with node.attributes?.some((a) => a.name === 'aria-hidden'). However, technically aria-hidden="false" means the element is NOT hidden. Both the original and the PR treat any aria-hidden attribute as hidden regardless of value. This is a pre-existing issue in the original, so matching is fine.

5. Tests are solid with good coverage including duplicate titles, boolean false values, empty strings, and hidden iframes.

Scope Analysis (gjs/gts)

This rule only checks GlimmerElementNode for node.tag === 'iframe' -- HTML element inspection. No scope analysis needed.

Summary

The main concern is the divergent behavior for duplicate title detection -- the original flags both the first and second iframe when a title is duplicated, while the PR only flags the second. Also, the single missingTitle message is used for all error conditions (missing, empty, duplicate, boolean false), losing diagnostic specificity.

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