Fix WAL TypeSpec build graph

[adsharma]

Summary

• keep WAL TypeSpec regeneration as an explicit target instead of a normal build dependency
• compile checked-in WAL record sources directly during ordinary builds
• fix the MSVC extension-build check to compare BUILD_EXTENSIONS as a string
• update WAL TypeSpec docs to describe explicit regeneration

Verification

• cmake -B build/release -G Ninja -DCMAKE_BUILD_TYPE=Release .
• cmake --build build/release --target lbug_storage_wal -j 2
• confirmed no generated WAL record files changed