bootstrap from 2026.9.24.1, and the verification that read the right file

[Sunrisepeak]

Summary

• .xlings.json: the bootstrap pin moves to 2026.9.24.1, which is released, mirrored and indexed:
  • release run 35924420072 green on all four platforms;
  • each archive and its .sha256 uploaded to GitCode by the local gtc as it appeared, GET 200 at the upstream size, and 8/8 byte-identical with the GitHub release; every archive matches its sidecar;
  • bump(mcpp): track 2026.9.24.1 as latest openxlings/xim-pkgindex#871 merged as 4440c503: the four hashes match the downloaded archives slot by slot, latest is exactly three lines naming 2026.9.24.1, and the index pointer's index_version is 4440c50. A SubOS sandbox with the CN mirror resolves latest -> 2026.9.24.1 on its first xlings update.
• The sandbox verification script's section E read compile_commands.json from target/, where mcpp does not write it, so it failed on both releases and the old release's failure was read as the expected one. It now reads the project-root file after each configure and checks the fingerprint by the number of build directories.
• The implementation plan records T10 to T12 and moves to landed, as does the analysis. The design stays active: the payload-contract and acceptance clauses of SPEC-006 are scheduled with the next LLVM toolchain batch.

Sandbox readings (SubOS v924, --sandbox, CN mirrors for xlings and mcpp)

	2026.9.24.1	2026.9.21.3
fails	0 (21 checks pass)	6
D. doctor names a planted fixincludes-frozen header	pass	fail
E. macos_deployment_target from Linux: triple, env override, fingerprint	pass (11.0, 12.0, 1 -> 2 directories)	fail (14.0, 14.0, 1 -> 1)
F. foo: refused; xim:msvc@system refused as a spelling	pass	fail (accepted; "only available on Windows hosts")
GUARD: install, new + run, gcc@15.1.0 <mutex>, index recipe fix, xim:gcc@16.1.0, mcpp-index consumers (yaml-cpp on gcc 16 and 15, CLI11, fmt on gcc and llvm)	pass	pass

The MSVC toolset selection is Windows-only and is covered by e2e 760 (Visual Studio 2026, MSVC 14.51.36231) and 239 on #688's CI.