The 351-vector corpus, and refuse_if_failed

The conformance runner discovers and runs the full shared corpus, 351 vectors across every type, through the read, write and measure paths, and refuses on any failed vector rather than skipping it (#70, #71, #72). refuse_if_failed is now public API: a defaulted Stream method, so an outside implementation can halt a stream on the first failed read (#71, #72). A zero-bit read is carried through the failure gate like any other width (#71).

The live interop job pins the C++ reference at v1.16.2 and exchanges bytes both ways (#68, #72).