Close the stale dead-session opacity backlog item with verified probe coverage

ROADMAP #38 stayed open even though the runtime already had a post-compaction
session-health probe. This change adds direct regression tests for that health
probe behavior and marks the roadmap item done. While re-running the required
workspace verification after a remote rebase, a small set of upstream clippy /
compile issues in plugins and test-isolation code also had to be repaired so the
user-requested full fmt/clippy/test sequence could pass on the rebased main.

Constraint: User required cargo fmt, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace before commit/push
Constraint: Remote main advanced during execution, so the change had to be rebased and re-verified before push
Rejected: Leave #38 open because the implementation pre-existed | keeps the immediate backlog inaccurate and invites duplicate work
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Reopen #38 only with a fresh compaction-vs-broken-surface repro on current main
Tested: cargo fmt --all --check; cargo clippy --workspace --all-targets -- -D warnings; cargo test --workspace
Not-tested: No live long-running dogfood session replay beyond the new runtime regression tests

rust/crates/plugins/src/lib.rs

@@ -7,6 +7,7 @@ use std::fmt::{Display, Formatter};
 use std::fs;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
+use std::sync::atomic::{AtomicU64, Ordering};
 use std::time::{SystemTime, UNIX_EPOCH};
 
 use serde::{Deserialize, Serialize};