fix(test): serialize tests that mutate CORTEX_MAX_TOPOLOGY_BYTES

[ammar-s847]

Summary

CI failed on disk::tests::topology_round_trips_on_disk with:

thread 'disk::tests::topology_round_trips_on_disk' panicked at src/disk/mod.rs:395:41:
called `Result::unwrap()` on an `Err` value:
  TopologyTooLarge { path: ..., bytes: 209, limit: 16 }

Root cause

topology_size_limit_enforced mutates process-global state — it sets CORTEX_MAX_TOPOLOGY_BYTES=16 to force the size guard to fire, then unsets it. cargo test runs tests in parallel by default, so when topology_round_trips_on_disk happens to call read_topology during that window it observes the 16-byte limit and trips the guard against the real ~209-byte topology file.

Local runs happened to schedule the tests in an order that avoided the race; fresh CI hit it.

Fix

Module-level Mutex<()> that both env-touching tests acquire before reading or writing the env var. Lock is poison-safe via unwrap_or_else(|e| e.into_inner()) so a panic in one test can't permanently wedge the rest.

No production code change — the env var contract and the production read path are unchanged.

Test plan

• cargo test -p hebb --features disk — green
• Ran the disk tests 5× in a loop — both ordering combinations pass
• Wait for CI green