Preserve descendant subtrees when reprioritizing

[AryanRG]

Reprioritizing a stream beneath one of its descendants currently promotes the descendant's children to its old parent while leaving them in the descendant's child list and scheduling queue. In RFC 7540 section 5.3.3, Figure 5, moving A beneath D incorrectly leaves F attached to C instead of D. Exclusive reprioritization also leaves duplicate child ownership.

Pass strip_children=False when moving the descendant to avoid the cycle. This preserves the subtree until the existing exclusive/nonexclusive insertion logic runs. The regression covers direct and indirect descendants, both exclusive modes, parent pointers, child lists, scheduling queues, and scheduling output.

Validation:

• Unchanged base 532ace514de40ebf9a9c0a4418cbfb0e72c8f7a4: 170 tests passed with 100% coverage; all four new regression cases fail against that base.
• Fixed: 174 tests passed on Python 3.10.22 and 3.14.4, both with 100% statement and branch coverage.
• Flake8, mypy 0.910 with the repository's pytest <7 constraint, Sphinx HTML build, check-manifest, sdist/wheel builds, and twine check passed.
• Black 26.10.1 still flags existing blank lines in src/priority/__init__.py and the imports in test/test_priority.py; the same formatting failures reproduce on the unchanged base. No added lines are flagged. The remaining interpreter matrix was not run locally. Hosted CI is awaiting maintainer approval; no hosted jobs have run.

AI assistance was used to prepare implementation and regression tests.