fix(mtmharness): harden Modern Go wrapper cache

[codeh007]

Summary

• validate the pinned Modern Go CLI version before executing an existing cache entry
• use private, collision-resistant temporary paths for POSIX and PowerShell installs
• escape the bundled PowerShell wrapper path and correct the pinned CLI version attribution
• add an offline regression test for lazy install, cache reuse, and corrupted-cache refusal

Verification

• pnpm --filter mtmharness run check: 22 Vitest files / 84 tests and 24 TAP tests, all passing
• pnpm --filter mtmharness run pack:check: tarball contract passing with wrapper resources included
• POSIX wrapper sh -n: passing
• real wrapper probe against /workspace/gomtm/go.mod: 54 guidelines and explain sync_waitgroup_go succeeded
• PowerShell runtime probe: unavailable in this Linux environment; source reviewed statically

Implements the follow-up correction for codeh007/gomtm#667. The earlier version-output concern was checked against upstream CLI source and real output: --version intentionally returns exactly v0.1.1, so that finding is not included as a code change.

[coderabbitai]

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Run ID: e4da1585-2428-443c-b542-a88317ae1b9e

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