sdk-builder/ compiles the pico-sdk once per board into platform/system/<board>/ (libpico.a, merged headers, flag files), and that output is committed.

• Arduino builds stay fast and need only the ARM compiler; no CMake for users.
• Clone and go: the IDE works from a fresh checkout, even years later, without rebuilding the SDK on a matching toolchain.
• Flag lists are generated from the SDK's own CMake (via a "probe" target) instead of hand-copied into platform.txt, which was the main maintenance pain in the first attempt (C:\repos\arduino-phyduino).
• An SDK update shows up as a reviewable diff of compile_flags.txt / link_flags.txt.

About 9 MB per board in git, regenerated on SDK updates. Generated files must never be hand-edited (GENERATED.md in each bundle says so).