D001 - A host build beside the target build

Status: decided Date: 2026-09-26

One source tree builds both the target shapes and make host, a native binary with every platform function stubbed or backed by the host C library. The host build is required and kept building.

Why: without it the first place the harness runs is inside an unfinished emulator, where a harness bug and a platform bug look the same. On the host the expected outcome is known, so the harness is falsifiable on its own, and a check that has never passed a known-good implementation is not evidence.

Rejected: target builds only - leaves the harness untestable except against the thing it measures.