chore(benchmarks): repoint dead /mnt-1 paths at /var after #3159 decommission (#3162)

[Xore]

Why

/mnt-1 was decommissioned in #3158/#3159. Every remaining default across the
benchmark/training tree still pointed at the dead mount, so scripts would
silently write to the root fs instead of failing loud on a missing volume.

What

• Round-7 corpus/training scripts: /mnt-1 defaults → /var
• Disk-layout docs and BLOB-RETENTION-POLICY rewritten for the single-volume /var layout
• unsloth compose bind mount repointed (only live-breaking mount in the set)
• build-supervisor.sh uses the real /var path
• injection-gate-protocol doc repointed
• rex86 large-model handling moves to a /var subdir (no second volume to split across anymore)
• Dated historical records (benchmark run logs, plan docs from before the decommission) left referencing /mnt-1 as-is, with a note — they describe what actually happened at the time

Closes #3162

[strix-security]

Strix Security Review

No security issues found.

_{Updated for 7822e5d.}

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[strix-security]

This PR is a mechanical chore that repoints decommissioned /mnt-1 paths to /var across benchmark/training shell scripts, Docker Compose bind mounts, a Python path constant, and supporting documentation. Every changed file was reviewed. The changes are path-string substitutions with no new input sources, sinks, or removed security controls. The single behavioral edit — build-supervisor.sh's DIR variable becoming environment-overridable via ${DIR:-...} — is a standard configurability idiom whose value is only used to locate a trusted build script in an operator/CI-run context, with no evidence of untrusted reachability. No authentication, authorization, injection, secret-handling, or file-path security regressions were introduced.

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