feat: persist PTY output activity evidence - #5

Problem

pty-rust is the Rust implementation of the same PTY session/registry contract as compoundingtech/pty. Node PTY #168 adds launcher-agnostic lastOutputAtMs; without parity, st2 session-fidelity observation depends on which PTY backend owns the session.

Goal

Persist the newest PTY output timestamp with the same camelCase metadata field, bounded live write rate, and retained-exit guarantee as PTY #168.

Decisions

• Stamp nonempty DaemonMsg::PtyData in the existing single-threaded actor; no second observer.
• Use actor-owned last_output_at_ms and recv_timeout for a trailing one-second persist deadline.
• Persist through the existing atomic metadata replacement; retained exit writes the final in-memory stamp in the exit mutation.
• Optional lastOutputAtMs is evidence only. pty-rust does not classify active/idle or know any launcher/harness.
• Create the repository's initial VRS hierarchy (docs/vrs/{requirements,spec}.md) with explicit user authorization.

Verification

• cargo check — green with Nix Rust + required Zig 0.15.2.
• cargo test --test cli_e2e --test registry_liveness — 16/16 green.
• Real-daemon proofs: absent before output, recent/advancing stamp across bursts, immediate output+exit retains the final stamp.
• axe vrs check --profile strict docs/vrs — green.

Build invocation used the documented generated-dependency checkout bypass because libghostty's build script checks out Ghostty inside Cargo target/ and the megarepo wrapper otherwise mistakes it for a canonical branch mutation:

MEGAREPO_ALLOW_CANONICAL_MUTATION=1 \
  nix shell nixpkgs#cargo nixpkgs#rustc nixpkgs#zig_0_15 -c cargo test ...

Complexity

Two actor fields (Option<u64>, Option<Instant>), one recv_timeout branch, one additive metadata field. No process, stream, writer thread, or per-chunk filesystem operation.

Concerns

The repository has no native devshell/CI. Building libghostty requires Zig 0.15.2 exactly; nixpkgs zig currently resolves to 0.16 and fails, while zig_0_15 is correct.

Friction & bottlenecks

• No repository-native Nix/CI/toolchain bootstrap.
• libghostty build checkout triggers the canonical-worktree policy unless the generated dependency checkout is explicitly authorized.
• Rustfmt over the whole repository creates large baseline churn; only semantic hunks were retained.

Follow-ups

• Keep the PTY Node/Rust compatibility matrix and VRS aligned as storage fields evolve.

References

• feat: persist last PTY output timestamp pty#168
• feat: compose driver and PTY session observation fidelity st2#353
• Define Node compatibility contract and export an embeddable PTY client crate #1