rules(rust): CPU parallelism audit items from the placement design

rules/common/performance.md

@@ -47,7 +47,8 @@ legacy tradeoff, a reference path, or a known migration target.
 - GPU reductions must not hide unbounded serial work in one thread. Use a
   documented strategy and threshold where a unit-thread fallback is allowed.
 - CPU threading policy has one source of truth per repository or runtime
-  object.
+  object. For Rust crates the placement design and its audit items are in
+  [`../rust/performance.md`](../rust/performance.md), "CPU Parallelism".
 
 ## Evidence
 

rules/index.md

@@ -39,7 +39,8 @@ applicable workflow and language rules.
 - [`rust/index.md`](rust/index.md): entry point; unsafe boundary, unit-test
   organization, debug/enum hygiene.
 - [`rust/performance.md`](rust/performance.md): allocation, slicing, linalg,
-  GPU kernels, uninitialized/scratch acquisition, build profiles, threading.
+  GPU kernels, uninitialized/scratch acquisition, build profiles, CPU
+  parallelism (the placement token and the spindle boundary, as audit items).
 - [`rust/numerical.md`](rust/numerical.md): numerical correctness, AD
   validation, typed-error classification.
 - [`rust/build-artifacts.md`](rust/build-artifacts.md): where Cargo and compiler

rules/rust/performance.md

@@ -126,15 +126,53 @@
 - Add regression coverage for both contracts: the uninitialized path stays
   unsafe or `MaybeUninit`-typed, and the zeroed path is safe.
 
-## Threading Principles
-
-- One repository source of truth for parallel thresholds (a declared policy
-  mechanism), not per-op copies of the same decision.
-- Short-circuit to the serial kernel when the effective thread count is one.
-- Library kernels must not reach for an ambient global thread pool outside the
-  declared policy mechanism. Non-provider parallel work executes within the
-  repository/runtime-owned execution context, so thread policy (including
-  one-thread serial execution) is decided at that single source of truth.
-- Provider-owned threading (BLAS/OpenMP-style) is controlled by provider
-  variables documented per repository; library code must not derive its own
-  pool policy from an ambient runtime.
+## CPU Parallelism
+
+The design is
+[`tensor4all-meta/docs/design/core-boundary-and-execution-ownership.md`](https://github.com/tensor4all/tensor4all-meta/blob/main/docs/design/core-boundary-and-execution-ownership.md),
+section "Where placement is decided" (its rules 1 to 13 and "faer and spindle"),
+with the walkthroughs
+[`cpu-placement-token.md`](https://github.com/tensor4all/tensor4all-meta/blob/main/docs/design/cpu-placement-token.md)
+and [`spindle.md`](https://github.com/tensor4all/tensor4all-meta/blob/main/docs/design/spindle.md).
+The design is the source of truth; the items below are what an audit checks.
+They apply to code that runs on `strided_exec::Exec`. In a repository not yet
+migrated to it, report each departure as a migration gap with its location,
+not as a defect.
+
+- **Fan-out only through the token.** Outside `strided-exec`, no crate calls
+  `rayon::scope`, `join`, `spawn`, `par_iter` / `par_chunks`,
+  `ThreadPool::install`, or, in kernel crates, `std::thread::spawn` /
+  `scope`. Check: `cargo deny` bans `rayon` except through the listed
+  wrappers, and clippy `disallowed-methods` lists the thread calls.
+- **No ambient pool.** A serial token runs on the calling thread and nothing
+  falls back to the global Rayon pool. Check: each consumer keeps a
+  `strided_exec::ledger::global_pool_unstarted()` test in its own process.
+- **Decide before entering.** Choose `jobs` and `grain` from `budget()` and
+  run a budget of one inline; never enter a pool to find out whether
+  parallelism pays.
+- **Budgets only shrink.** A token is not cloned or stored in plans and caches,
+  a callee does not re-derive or raise a budget, and a split hands each child
+  a token whose budgets add up to the parent's (`fork`, `for_ranges`,
+  narrowing with `limit`, sequential phases with `reborrow`).
+- **One source of truth for policy.** Parallel thresholds, `WorkModel` and
+  `BatchPolicy` live once, in the provider crate; per-op copies of the same
+  decision are a finding, and policy code starts no tasks.
+- **Vendor libraries through `Exec::external`.** faer and vendor BLAS receive a
+  plain width from `external`; no adapter derives a pool or thread count from
+  an ambient runtime. BLAS/OpenMP worker teams stay provider-controlled and
+  are documented per repository.
+- **spindle stays inside t4a-faer.** No other crate depends on spindle or opens
+  a `with_lock` region (check: `cargo deny` with `wrappers` listing only
+  `t4a-faer`). A batched caller such as tlinalg gives every slot to the batch
+  with `Par::Seq` items when the batch is at least the budget, and otherwise
+  forks the token with one `external` per item. Inside t4a-faer, every
+  parallel kernel reachable from a region dispatches through spindle; plain
+  Rayon code there runs on one thread.
+- **No Rayon type in a public signature.** Check: `cargo public-api` of
+  `strided-exec`, `tcontract-exec` and `tenferro-cpu` contains no `rayon::`.
+- **Barrier-bearing kernels use `broadcast`.** A refusal takes the
+  barrier-free path; the participant count is never clamped silently.
+- **Concurrency claims need the ledger.** A claim that work stays within a
+  budget is shown with the `strided-exec` ledger's peak at the root token, or,
+  inside faer or a vendor library, measured at the kernel boundary. A worker
+  index or a pool size is not evidence.

skills/tensor4all-rules-audit/SKILL.md

@@ -15,6 +15,8 @@ An audit request does not by itself authorize implementation, issues or PRs.
    files. Read applicable rules and referenced decisions, not the entire workspace
    by default. A local or uncommitted target is valid; describe it accurately.
    Do not require a push or a GitHub-visible commit to inspect local work.
+   Code that fans out on the CPU (Rayon, `strided-exec`, faer, spindle) is
+   audited against `rules/rust/performance.md`, "CPU Parallelism".
 2. **Inspect the real path.** Read enough source to understand the contract,
    callers and existing checks. Use existing API documentation when helpful;
    do not build release-mode documentation as a default audit prerequisite.