audit(standards): zero concurrency model checking + unsafe Sync impl under-documented + no stress tests

[forkwright]

Summary

Three concurrency safety gaps found:

1. No loom or Miri — zero model-checked concurrency

No crate declares `loom` as a dependency, no `#[cfg(loom)]` gates exist, and Miri is not referenced anywhere. The four `unsafe impl Send/Sync` declarations are unverified by any formal tool.

2. MmapVectorStorage Sync impl is under-documented

`crates/krites/src/runtime/hnsw/mmap_storage.rs:85-86`:
```rust
unsafe impl Send for StorageInner {}
unsafe impl Sync for StorageInner {}
```

`as_bytes(&self)` dereferences a raw `*mut u8` from mmap under `&self`. The comment says "access is controlled through `&self` / `&mut self`" but does not name the external guard (per-relation `ShardedLock` in `Db`) or prove that concurrent `push(&mut self)` + `get(&self)` races are excluded.

The other two `unsafe impl Sync` declarations (`FjallReadTx`, `FjallWriteTx` at `krites/src/storage/fjall_backend.rs:144,149`) have adequate inline rationale.

3. No stress tests — shallow concurrent test coverage

• `SessionStore` (`Arc<Mutex>`) has 18 production clone sites across nous/pylon/diaporeia but zero concurrent access tests
• Integration concurrent tests are N=2 with `tokio::join!` only — no iteration, no write-write contention
• No pattern of `for _ in 0..N { spawn(...) }` exists anywhere
• nextest has no concurrency profiles

Contention surface (45 Arc<Mutex/RwLock> sites)

Crate	Count	Hot path?
nous	18	Yes — SessionStore
pylon	6	Yes — same SessionStore
krites	6	Yes — running_queries, HNSW
koina	6	No
Others	9	Mixed

Fix priority

1. Document the external guard for `MmapVectorStorage` Sync impl (or add a lock)
2. Add stress test for `SessionStore` concurrent access (spawn 100 tasks, mix reads/writes)
3. Evaluate loom for the krites transaction/storage layer
4. Add multi-thread flavor to integration concurrent tests

Source

Concurrency stress testing audit — basanos TESTING.md + RUST.md concurrency sections.