[Janus] P1: repack the wasi-threads toolchain artifact with the box's clang builtin headers (v2 grafts musl headers; wint_t/wchar_t undeclared)

[TJKoury]

Graph task: sdk-isomorphic-toolchain-sysroot-artifact · state: open · priority: 1 · components: [module-sdk, sdn]
Agent: Janus

Defect

space-data-network kubo/sdn/flowcc/toolchain/build-threads-bits.sh:75-84
grafts iso646.h stdalign.h stdarg.h stdbool.h stddef.h stdnoreturn.h tgmath.h __stddef_max_align_t.h from the emscripten sysroot's top-level include into
sysroot-wasi-threads/include/. Those are musl libc headers. clang's builtin
include dir (the one that carries __need_wint_t / __stddef_wint_t.h /
__need_wchar_t) is never staged, and the box's driver emits
-internal-isystem lib/clang/16.0.0/include — a RELATIVE path with nothing
behind it in the box FS. Measured search list inside the box:

#include <...> search starts here:
 /sysroot/include/wasm32-wasip1-threads
 /sysroot/include
End of search list.

Result: wctype.h:27: error: unknown type name 'wint_t' and
stdlib.h:85: error: unknown type name 'wchar_t'. Every TU that reaches
<wctype.h> or <stdlib.h> — i.e. essentially every C++ TU and most C TUs —
cannot be compiled by the boxed toolchain. The comment at
build-threads-bits.sh:12-16 blames clang-16 and libc++; the real cause is
this graft. flow_runtime.threads.o ships as a prebuilt native object because
of it.

Required

1. Stage the box's OWN clang builtin include dir at the resource-dir path the
   box driver reports (clang -print-resource-dir → lib/clang/<ver> — treat
   the relative path as a defect too and pass -resource-dir explicitly, as
   wasiThreadsToolchain.js:133-136 already does natively).
2. Repack as toolchain v3: new SHA256SUMS with a
   CLANG_BUILTIN_INCLUDE_ROLLUP_SHA256 alongside the existing
   WASI_THREADS_SYSROOT_ROLLUP_SHA256; stage-toolchain.sh asserts it.
3. Guardrail test: compile a C++17 TU that includes <vector> <string> <cmath>
   AND a C TU that includes <stdlib.h> <wctype.h> through the box; both must
   emit an object. Run it in the SDK toolchain suite, not only in kubo.
4. Re-evaluate whether flow_runtime.threads.o still needs to ship prebuilt.
   If the box can compile it, the bake stops shipping a native-built object as
   staged data and the isomorphism argument gets simpler.

Evidence

lab/isomorphic-toolchain/cxxfix.mjs and bigcxx.mjs (SDK branch
janus/isomorphic-toolchain-spike, commit ccac68d) reproduce both the
failure and the fix. With builtin headers grafted:
foundation/orbits/src/orbits_module.cpp (177 KB C++17 + flatbuffers) compiles
in 2,629 ms cold / 1,531 ms warm, 57.6 MB guest heap, deterministic across
runs, object 125,806 B vs native 22.1.2's 124,082 B.

Cross-repo: the artifact lives in space-data-network, the consumer contract is
the SDK's. Coordinate with Hermes before touching kubo/sdn/flowcc/toolchain/.
Mirrored from the dev graph (graph/tasks/sdk-isomorphic-toolchain-sysroot-artifact.md in DigitalArsenal/spacedatanetwork-stack); closes automatically when the graph task completes.