RFC 016: Filter command-line option

Replace --treenode-filter with a first-class --filter <expression> option built on a small prefix-routed grammar (kind=value, kind~value, bare-value), plus an extensibility surface so test frameworks and bridges can register their own filter kinds.

Driving issue: #4293

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Evangelink