Preserve text gates in renamed literal-content elements

[goutamadwant]

Summary

Fixes #482.

When an ElementPolicy renamed a container to an element such as style, a
nested allowed element could become the policy's active text container even
though the renderer could not emit its tag inside literal content. That let the
nested text bypass the outer element's text gate.

This change tracks whether a kept literal-content element is open separately
from whether its text is allowed. Nested start tags are treated as dropped
before they can establish another text gate, so their text remains governed by
the outer element. The state is restored when that element closes.

Tests

• Added negative coverage showing nested text is removed when the renamed
  literal-content element does not allow text.
• Added positive coverage showing explicitly allowed text still survives while
  the nested tag is removed.
• Verified a following sibling is unaffected after the literal-content element
  closes.
• Verified change reporting treats the nested tag as discarded without
  separately reporting its attributes.
• ./mvnw -ntp -B clean verify passes on JDK 25: 604 library tests and 7 example
  tests.