[Nas01010101]

Fixes #2416

__scope.nest() replaces the stop token in the receiver's environment with the scope's token, so a stop request on the start_now storage never reached the senders.

This change wraps each sender in __stop_when with the storage's token before nesting it. The sender then responds to both the storage and the scope. __counting_scopes.hpp and __spawn_future.hpp use __stop_when the same way.

Tests

Added to test/exec/async_scope/test_start_now.cpp:

• start_now request_stop stops the senders fails without the change in start_now.hpp.
• start_now senders still stop with the scope passes with and without it, and covers the scope's stop request.

test_start_now.cpp is the only file that includes start_now.hpp. That test file passes on:

Compiler	Flags
clang 21.1.8	-O0 -Wall -Wextra -Werror
clang 21.1.8	-O0 -fsanitize=address,undefined
clang 21.1.8	-O1 -fsanitize=thread
clang 21.1.8	-O2 -std=c++23
g++ 13	-O0
g++ 14	-O0 -fsanitize=address
g++ 15	-O2 -std=c++23

A separate program stopped a sender running on static_thread_pool, 300 times through the storage and 300 times through the scope, with no reports under TSan or ASan/UBSan.

test.exec built with CMake (clang 21.1.8, Debug, C++20) passes all 507 test cases. libc++ and nvc++ were not run locally.

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