feat(engine): shared rayon pool for the disjoint-parallel core

Replace the per-call std::thread::scope fan-out in run_indexed with one
process-wide rayon pool. run_indexed's callers nest — sweep (grid
points), monte_carlo (seeds), and walk_forward (window bounds), where a
walk_forward window runs an inner sweep — and each std::thread::scope
spawned its own available_parallelism() OS-thread batch, so the two
levels multiplied to ~available_parallelism()^2 threads (measured
150-240 concurrent on a 24-core box during a 42-cell campaign, ~10x
oversubscription). rayon's pool is process-wide and persistent: a nested
par_iter work-steals within the same N workers instead of spawning, so
live workers never exceed the pool size.

Determinism (C1) is preserved bit-for-bit: run_indexed collects via
IndexedParallelIterator in enumeration order (not completion order), and
the float aggregation (summarize_r / McAggregate / stitch) runs after
the ordered collect, so IEEE-754 non-associativity introduces no
variance. The existing _with_threads(1) == _with_threads(8) == public
determinism tests stay green, now driving local rayon pools of 1 and N
workers via ThreadPool::install.

Structural inversion: the public sweep/monte_carlo/walk_forward become
the core (calling run_indexed on the ambient pool through a shared
assemble_* helper); the _with_threads variants collapse into thin,
test-only wrappers (#[cfg(test)]) that run the same assembly inside a
local pool. run_one stays Fn + Sync (no + Send): run_indexed borrows it
in the map closure and the wrappers borrow it into install — passing it
by value would move it into rayon's Map and force F: Send, a bound the
public callers do not carry (hence the load-bearing
#[allow(clippy::redundant_closure)] on run_indexed).

Adds a nested-oversubscription regression test (asserts nested
run_indexed never exceeds the pool size — the pre-rayon shape ran
POOL^2) and an ignored wall-time benchmark. rayon admitted under the
amended-C16 per-case dependency policy; it stays within C1 (parallelism
across sims, never within one).

This is the shared pool only — the first of three steps toward
saturating the cores. The diagnosis found the larger loss is the idle
valleys (~42% of the box idle: the sequential cell loop and the
single-threaded bootstrap), which the pool alone does not fill. Two
follow-ups are filed and recorded on #268: the Registry write path is
not concurrency-safe (append_family races on a shared families.jsonl),
the prerequisite for parallelizing the C1-disjoint cell loop with a RAM
budget bounding concurrently-active instruments (the FileCache retains a
symbol's parsed files while any reference is live).

Verified: cargo test -p aura-engine (276 passed, the C1 determinism
tests green), the new oversubscription test green, the benchmark runs
(no overflow), clippy -D warnings clean, cargo test --workspace no
failures.

closes #268

Cargo.lock

@@ -168,6 +168,7 @@ dependencies = [
  "aura-std",
  "chrono",
  "chrono-tz",
+ "rayon",
  "serde",
  "serde_json",
 ]
@@ -404,6 +405,25 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "crossbeam-deque"
+version = "0.8.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5181e0de7b61eb03a81e347d6dd8797bae9da5146707b51077e2d71a54ec0ceb"
+dependencies = [
+ "crossbeam-epoch",
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "crossbeam-epoch"
+version = "0.9.20"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2d6914041f254d6e9176c01941b21115dcfb7089e55135a35411081bd106ef3f"
+dependencies = [
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "crossbeam-utils"
 version = "0.8.21"
@@ -479,6 +499,12 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "either"
+version = "1.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91622ff5e7162018101f2fea40d6ebf4a78bbe5a49736a2020649edf9693679e"
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -803,6 +829,26 @@ version = "5.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "69cdb34c158ceb288df11e18b4bd39de994f6657d83847bdffdbd7f346754b0f"
 
+[[package]]
+name = "rayon"
+version = "1.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fb39b166781f92d482534ef4b4b1b2568f42613b53e5b6c160e24cfbfa30926d"
+dependencies = [
+ "either",
+ "rayon-core",
+]
+
+[[package]]
+name = "rayon-core"
+version = "1.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22e18b0f0062d30d4230b2e85ff77fdfe4326feb054b9783a3460d8435c8ab91"
+dependencies = [
+ "crossbeam-deque",
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "regex"
 version = "1.12.3"