docs: preserve falsifiable ExecSurface CI stdout privacy boundary - #169

Why

ExecSurface #164 records a successful first zero-assistance trial plus a separate CI-log privacy concern. The upstream trial harness currently inherits stdout/stderr for its observe subprocess, unlike its helper run_text. This makes inspect-before-sharing insufficient for path-bearing bytes printed during observation.

What

• Pin the exact source-level boundary and an independently runnable synthetic subprocess reproduction.
• Specify a bounded regression contract for stream containment without changing evidence semantics or qualification state.
• Keep explicit non-claims: this is not an upstream fix, not an executed ExecSurface regression, and not an externally qualified trial.

External intake: AETHERXGLOBAL/execsurface#164

Next verification

Run the synthetic probe and seek upstream owner confirmation before proposing a producer-side patch. Do not publish raw typed-evidence streams.