dash: MSVC-portable coinbase PPLNS payout muldiv (fixes c2pool-dash Windows v0.2.1)#709
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…ash Windows) coinbase_builder.hpp evaluated the PPLNS per-script payout proportion (worker_payout * weight / total_weight, and the pre-v36 *49/(50*total) arm) in a native unsigned __int128 intermediate. __int128 is a GCC/Clang extension MSVC does not provide (error C2065: __uint128_t undeclared identifier), so the c2pool-dash Windows binary failed to compile in the v0.2.1 release build while Linux/macOS DASH passed -- the last DASH gap for the all-5-coin v0.2.1. Same class of fix already shipped for BCH abla (#688) and DGB arith256 (#690): extract the 128-bit muldiv into dash::payout::payout_share (payout_muldiv.hpp) and guard it with #if defined(__SIZEOF_INT128__). The native path keeps the __uint128_t arithmetic BYTE-EXACT for the shipping Linux/macOS packages; MSVC falls back to boost::multiprecision::uint128_t (header-only, already a c2pool dependency) which yields bit-identical results. coinbase_builder.hpp:131-138 is rewired through it; no payout amount changes on any platform. This is CONSENSUS PAYOUT math -- a one-satoshi divergence forks payouts -- so test_dash_coinbase_muldiv is the required guard: layer A pins cross-platform known answers (incl. products > 2^64) that MSVC can run without a native to diff against; layer B (#if __int128) proves native == portable across the DASH payout operand grid plus a 200k-vector deterministic fuzz, over both the v36 and pre-v36 branches. Wired into test/CMakeLists.txt and both build.yml --target allowlists. Verified locally: test_dash_coinbase_muldiv 5/5 PASS (native==portable over 200000 fuzz vectors); test_dash_coinbase_parity 10/10 unchanged.
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Problem
DASH is the only coin whose Windows binary fails to compile in the v0.2.1 release build (Linux + macOS DASH are green) -- the last DASH gap for the all-5-coin v0.2.1. Root cause (MSVC run 29383023622):
```
src/impl/dash/coinbase_builder.hpp(131): error C2065: __uint128_t: undeclared identifier
```
The PPLNS per-script coinbase payout proportion was evaluated in a native `unsigned __int128` intermediate (`worker_payout * weight / total_weight`, and the pre-v36 `49 / (total50)` arm). `__int128` is a GCC/Clang extension MSVC does not provide.
Fix (mirrors BCH abla #688 / DGB arith256 #690)
Wired into `test/CMakeLists.txt` and both `build.yml` `--target` allowlists.
Local verification
Consensus surface: payout arithmetic is bit-identical to what merged; the only behavioural change is that MSVC now compiles.