[M0-CORE-05] Fix Windows CI: C4324 in pools_tests (alignas(16) padding is fatal under /WX)

[offdev]

Problem

The windows-msvc job fails compiling tests/laige-core/pools_tests.cpp:

pools_tests.cpp(106,1): error C2220: the following warning is treated as an error
pools_tests.cpp(106,1): warning C4324: '...::Padded': structure was padded due to alignment specifier

MSVC C4324 fires when an alignment specifier pads the struct, and the engine policy (/W4 /WX, NFR-8.10) makes it fatal. The Padded fixture (float a; char b; + alignas(16)) reached its 16-byte size purely through that padding — natural member size is 8 B.

Fix

Restructure the fixture to four floats. The observable properties are unchanged:

• sizeof(Padded) == 16 and alignof(Padded) == 16 exactly as before, so the ElementSlot stride the fixture pins (max(sizeof(T), alignof(T))) is unchanged;
• the members now fill the struct, so alignas(16) contributes alignment without adding padding and C4324 cannot fire;
• no warning suppression (CORE-010).

The fixture's purpose (an element with non-trivial alignment pinning the bytes accounting) is preserved; a comment now documents the C4324 constraint so the fixture is not accidentally shrunk again.

Validation

• GCC build of laige-core_tests warning-clean (-Wall -Werror);
• full shared executable: 135/135 tests pass, including PoolStats.AlignedElementsUseAlignedStride (bytesInUse == 16, bytesCapacity == 2·(16+9));
• no other code references the fixture; no include-graph or dependency impact.