ExtraTests X21 and X27 defined their listeners at namespace scope, which
clang rejects under -Wweak-vtables with -Werror on Linux and macOS; they
are now in anonymous namespaces.

After a fatal signal Catch2 closes the open sections itself, so on POSIX a
crash was reported outside the section it happened in. The scheduler now
ignores section ends that follow a fatal condition.

New unit tests cover the planner (every selection, dependency closure and
cycles, hash and duration shards, incremental runs, learned time limits,
ordering, retries, quarantine and replay), every command-line option and
its errors, and the failure analysis (history classes, signature groups,
crash and hang locations, changed files, replay commands).
Catalog::FromEntries builds a catalog without a test registry so the
planner can be tested on its own.