fix: deduplicate machine-assigned tags

[casavaca]

Fixes #897.

Every section in a manual carries a tag: it becomes the section's HTML id and the target that cross-references resolve to, and Verso generates one from the title when the author has not chosen it. Two sections whose titles agree therefore need two different generated tags. They do not get them, and the manual fails to build instead:

# Same
First.

# Same
Second.

Duplicate tag '_private.VersoManual.Basic.0.Verso.Genre.Manual.Tag.internal "Test--Same"'

The generator, freshTag, is meant to prevent exactly this: it tries Same, then Same-0, until it finds a name that TraverseState.tags does not already hold. That search never finds anything. It looks the candidate up as a String, which Coe String Tag turns into Tag.provided, while the name it goes on to store is a Tag.internal, so it never sees the names it assigned itself and the numeric suffix is never appended.

The search was right when the suffix was introduced in e223cfc, where tagStr returned a Tag. It stopped working in e842684, which changed freshTag to return a String: the insertion was updated to Tag.internal tag, the lookup kept its old shape, and the coercion kept it compiling. Since then any two headings that sluggify to the same string collide. Headings in a non-Latin script collide readily, because every unsupported character becomes ___: two Japanese headings of the same length always produce the same slug.

This changes the lookup to use the constructor that is stored, and adds two tests to VersoTests.Tags — two elements with the same hint, and two different Japanese headings. On the current main they produce Same/Same and ______/______; with this change they produce Same/Same-0 and ______/______-0, and the example above builds, giving the two sections Test--Same and Test--Same-0. lake test passes (649 tests).

This picks up #937, which proposed the same one-line change and was closed for lack of response — the failing test and the fuller description asked for there are what this adds.

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Drafted with Claude Opus 5