fix: document inert Linux ELF fixture for PTY test gate #2203

Priority: P1 | Area: runtime | Labels: fix,P1,area:runtime | Milestone: 0.0.24 | RFC: OQ-003 (ADR-0002 platform tiers), CTX-0267 PTY test-gate policy; CTX-1151 sandbox contract (#1984) | Task: CTX-1311

Problem

At origin/main revision 662212c, just pty-gate reports an ungated live-spawn marker at crates/bitty-sandbox/src/lib.rs:676 in the Linux-only elf_interp_finds_loader_for_dynamic_tools test. The test writes a shebang fixture and passes its path to linux::elf_interp; it never executes the fixture or opens a PTY. The existing checker accepts cfg(unix) but does not classify cfg(target_os = "linux") as a gate.

Scoped Fix

Add the existing audited per-hit pty-gate-exempt directive immediately above the inert fixture write, accurately stating that it is parser data and never executed. Change only crates/bitty-sandbox/src/lib.rs. Do not change the global checker, function behavior, platform cfg, or introduce a generic PTY skip.

Acceptance Criteria

• Reproduce the original gate failure before editing.
• just pty-gate, just fmt-check, and focused sandbox tests/Clippy pass.
• Preserve Linux-only cfg and every executable token.
• Use bounded package-only verification and remove the dedicated task cache.
• Stop at a managed local commit ready for independent review; no push, PR, merge, tag, or release.

Verification

• Baseline just pty-gate: failed at the expected fixture write.
• After the comment: just pty-gate and just fmt-check pass.
• just --command cargo test -p bitty-sandbox --locked -- --test-threads=1: 11 unit tests and 1 doctest passed, no failures/ignored tests.
• just --command cargo clippy -p bitty-sandbox --all-targets --locked -- -D warnings: passed.
• Build jobs limited to 2, dev/test debug set to 0, incremental disabled. Dedicated 15 MB cache removed and absence verified.
• Full workspace just check, Windows verification, and local CI are not claimed green; WSL/source and local-CI blockers are owned by separate tasks.