fix: harden tmux compatibility follow-up

[ictechgy]

Summary

• preserve non-UTF-8 tmux socket identity instead of comparing lossy display strings
• bound self-reexec path tests, derive exact test names, and require an explicit child sentinel
• harden the tmux user-option, nested-session, generation-safety, and mobile transcript fixtures identified by the post-merge review of Add bounded tmux user-option persistence and fix nested inventory #167
• add end-to-end pane-first OMX ownership read coverage

Validation

• cargo fmt -- --check
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --locked
• cargo build --release --locked
• cargo audit (passes with the repository's allowed transitive RUSTSEC-2026-0190 warning)

Scope

This is a normal Git follow-up and intentionally does not mutate the blocked historical Ultragoal ledger.