[CI only] validate released Jolt prerequisite stack

[casselc]

CI-only validation for the released-Jolt prerequisite stack.

Do not merge. This targets the existing codex/jolt-upstream-fork branch so Actions exercises the five candidate commits without opening anything against an upstream organization.

Local evidence:

• Fish environment, Jolt v0.7.27, pinned Chez 10.4.1
• exact published jolt-net 10542fc resolution with no local override
• required Hegel install checksum verified
• full acceptance suite plus 28 tests / 122 assertions and all six TCP properties passed
• actionlint clean aside from its stale runner-label table; GitHub currently documents windows-11-vs2026-arm as GA
• Claude read-only review: READY

This PR will be closed after CI evidence is collected.

[casselc]

Final validation passed on run 32972311952 at 42c72dd.

Green lanes:

• Linux x86_64 full runtime
• Linux aarch64 full runtime
• macOS arm64 full runtime
• macOS x86_64 full runtime with libhegel 0.33.0 built from source
• Windows x86_64 portable TCP contracts and buffer properties
• Windows aarch64 native source-mode buffer preview

The earlier red runs exposed and then verified fixes for Jolt v0.7.27 Windows drive/UNC path rebasing, relative gitlibs cache cwd mismatch, and the stale Intel-macOS libhegel 0.30.1 pin.

Closing this CI-only PR unmerged. The candidate branch remains available.

[casselc]

CI-only validation complete; closing unmerged as planned.