perf: parallelize `rw?` tactic

Use `MetaM.parIterWithCancel` to try all candidate rewrites in parallel
while preserving deterministic result ordering. When an rfl-closeable
result is found (and `stopAtRfl` is true), or the maximum number of
results is reached, remaining tasks are cancelled.

This removes the old sequential `takeListAux` implementation along with
the heartbeat-based early termination and `RewriteResultConfig` structure.

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Co-Authored-By: Claude <noreply@anthropic.com>