Add (an interface for?) some known segments that break the rules

E.g. P3P is a thing, even with the digit boundary.

Also: S3, HTML(4|5)

No file, test, or entry point is named. Start by reproducing how the current text object handles the examples P3P, S3, and HTML4/HTML5, then trace the existing segment rules and decide how known exceptions should be represented; done means those examples are handled without breaking ordinary variable segments.

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