Fix iterate subscribe exception specification

[fallintoplace]

What

• Make iterate report when range access or operation-state construction can throw during subscribe.
• Add a regression for a range whose begin() throws.

Why

iterate_t::subscribe() derives its exception specification from an internal helper, but that helper was unconditionally noexcept even though it calls ranges::begin() and ranges::end(). A throwing range could therefore terminate instead of propagating the subscription exception.

Test

• ctest --test-dir build --output-on-failure -j 8
• All 948 tests passed.

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