test(control_plane): fix flaky env-var data race in unit tests

[zzylol]

Summary

control_plane unit tests mutate process-global environment via std::env::set_var / std::env::remove_var. cargo test runs #[test]/#[tokio::test] functions on parallel threads within a single process, so these mutations raced:

• Three independent module-local mutexes (replan::tests::TYPED_ENV_GUARD, emit::stage_config::tests::MEMORY_LIMIT_ENV_LOCK) did not serialize against each other. Concurrent setenv/unsetenv (documented as unsound in a multithreaded program) raced on the one global environ table.
• ~50 env-reading tests (notably the mvp46_* routing-shape assertions) held no lock, so they could transiently observe a concurrent writer's ASAP_EDGE_FUSED=1 and assert against the wrong (fused vs. routing) emit shape.

Result: nondeterministic failures depending on thread scheduling (reproduced reliably: 6 / 2 / 4 random mvp46_* failures across three full --lib runs before the fix).

The racy variables are ASAP_EDGE_FUSED, ASAP_AGENT_MEMORY_LIMIT_MIB, and USE_TYPED_STAGE_SPLIT (the originally-filed ASAP_EDGE_FUSED-only framing was incomplete).

Fix

• Add a crate-wide #[cfg(test)] test_support module in lib.rs exposing one shared env_lock() mutex plus an RAII EnvVarGuard (set/unset on construct, restore the prior value on drop — panic-safe).
• Route every lib unit test that reads or writes a process-global env var through that single lock, regardless of module (emit/stage_config.rs, emit/mod.rs, emit/trait_def.rs, replan.rs). The three old per-module locks are removed.
• Production code is unchanged — this is purely a test-hygiene fix. main.rs keeps its own guard (it is a separate test binary and is already internally consistent).

No new dependency added (reuses the existing static Mutex pattern the crate already used in three places, now unified into one).

Test plan

• cargo test -p control_plane --lib x5 — 769 passed; 0 failed every run (was flaky before).
• cargo test -p control_plane (lib + bin) x3 — 769 + 28 passed, no failures.
• cargo build -p control_plane clean.
• cargo clippy -p control_plane --lib --tests — no new warnings from the changed files (pre-existing unused-import / vec_init_then_push lints in untouched production code remain).

🤖 Generated with Claude Code