Fixes #534

tf::find_if / tf::find_if_not called std::distance before checking whether the range is sentinel-based (e.g. std::views::iota(0) with std::unreachable_sentinel), which is undefined for infinite ranges.

Changes

• Detect sentinel ranges via !std::is_same_v<E_t, B_t> and fall back to sequential std::find_if / std::find_if_not.
• Keep existing parallel path for finite ranges.
• Add unit tests for sentinel-based iota ranges.

Test plan

• unittests/test_find.cpp — find_if.SentinelRange, find_if_not.SentinelRange