fix(stdexec): silence unused-parameter warning in __fuse_token_fn

[alwaysprince05]

Problem

Including <stdexec/execution.hpp> fails to compile on recent clang versions (e.g. Apple clang 21, Xcode 26) when the user builds with -Wall -Wextra -Werror:

In file included from repro.cpp:1:
include/stdexec/__detail/__stop_when.hpp:137:60: error: unused parameter '__rcvr_token' [-Werror,-Wunused-parameter]
  137 |       operator()(_SenderToken __sndr_token, _ReceiverToken __rcvr_token) const noexcept
      |                                                            ^
1 error generated.

Minimal repro (compiles with no stdexec API in use at all):

#include "stdexec/execution.hpp"
int main() {}

clang++ -std=c++20 -I include -Wall -Wextra -Werror -c repro.cpp

Root cause

The unstoppable-token overload of __fuse_token_fn::operator() in __stop_when.hpp takes the receiver's stop token but intentionally ignores it — when the receiver's token is unstoppable, the net token is just the sender's captured token. Because it is an uninstantiated function template at header-parse time, clang (unlike GCC) still diagnoses the unused parameter, and -Wunused-parameter is part of -Wextra.

CI misses this because add_compile_diagnostics() applies -Wall -Wextra -Wpedantic -Werror only for Clang/GNU CMake compiler IDs — not AppleClang — and the Linux clang versions used in CI don't fire the diagnostic for this template. macOS users on Xcode 26 toolchains hit it immediately.

A whole-tree scan with Apple clang 21 (-Wall -Wextra -Werror) shows this is the only remaining such warning across all public headers; every other #include compiles clean.

Fix

Mark the intentionally-unused parameter [[maybe_unused]], consistent with the existing convention in the codebase (e.g. exec/static_thread_pool.hpp, exec/sequence_senders.hpp, nvexec). No behavioral change.

Verification

• The minimal repro above now compiles clean with -Wall -Wextra -Werror on Apple clang 21 (Apple clang version 21.0.0); A/B-verified against pristine main.
• Whole-tree scan: all public headers (stdexec/execution.hpp, functional.hpp, coroutine.hpp, stop_token.hpp, concepts.hpp) compile warning-free with -Wall -Wextra -Werror on Apple clang 21.
• test.stdexec: all tests pass (623 test cases / 3536 assertions), normal configuration.
• test.exec: all tests pass (365 test cases / 3438 assertions), normal configuration.
• Same two suites built and run with STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON: all pass, identical counts — no interaction with the debug-connect machinery.
• clang-format --dry-run --Werror (clang-format 21) passes on the touched file.

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[alwaysprince05]

/ok to test

[ericniebler]

/ok to test 2ec64ce

[ericniebler]

thanks