feat(aura-std): add Mul + Sqrt primitives

The two genuinely-missing arithmetic primitives (Mul = two-stream f64 product,
Sqrt = one-input f64 root, negatives clamped to 0). They are the building blocks
for the volatility stop as a composition (rolling EWMA stddev), replacing the
fused VolStop node. Also corrects plan 0066 Task 3 (the VolStop removal must
migrate its stage1_r_e2e.rs caller — a false premise the implementer caught).

refs #117 #119

docs/plans/0066-stage1-r-primitives-volstop.md

@@ -218,13 +218,46 @@ Add `mod sqrt;` (alphabetical, after `mod sma;` / before `mod stop_rule;`) and
 
 ---
 
-## Task 3: Remove the fused `VolStop` node (keep `FixedStop`)
+## Task 3: Remove the fused `VolStop` node (keep `FixedStop`) + migrate its one caller
+
+`VolStop` IS referenced outside `stop_rule.rs`/`lib.rs`: `crates/aura-engine/tests/stage1_r_e2e.rs`
+imports it (line 30) and constructs `VolStop::new(1, 1.0)` in
+`r_is_stop_defined_two_stops_fold_to_different_expectancy`. That caller must be migrated
+in this task or the workspace build breaks.
 
 **Files:**
 - Modify: `crates/aura-std/src/stop_rule.rs`
 - Modify: `crates/aura-std/src/lib.rs`
+- Modify: `crates/aura-engine/tests/stage1_r_e2e.rs`
 
-- [ ] **Step 1: Delete `VolStop` from `stop_rule.rs`**
+- [ ] **Step 1: Migrate the `stage1_r_e2e.rs` caller off `VolStop` (FIRST)**
+
+`run_chain` drives a `&mut dyn Node` directly, so the vol stop (now a bootstrapped
+composite, Task 4) cannot slot in there. The test's property — "different stop distances
+fold to different R" — holds via two CONSTANT stops (R = (exit−entry)/distance, so a
+10×-tighter stop folds to a ~10× deeper R-loss). Replace the `VolStop` arm with a tight
+`FixedStop`:
+- line 30 import → `use aura_std::{FixedStop, PM_FIELD_NAMES, PM_RECORD_KINDS, PM_WIDTH, PositionManagement};` (drop `VolStop`).
+- in `r_is_stop_defined_two_stops_fold_to_different_expectancy`, replace the lines from
+  `let mut vol = VolStop::new(1, 1.0);` through the final `assert!` with:
+
+```rust
+    // A TIGHT FixedStop (distance 1.0) vs the wide one (10.0): R = (exit-entry)/distance,
+    // so the 10x-tighter stop folds to a ~10x deeper R-loss for a comparable drop.
+    let mut tight = FixedStop::new(1.0);
+    let tight_m = run_chain(&mut tight, &long_path(&[100.0, 100.0, 96.0]));
+    assert!(wide.n_trades >= 1 && tight_m.n_trades >= 1);
+    assert!(
+        tight_m.expectancy_r < wide.expectancy_r,
+        "stop choice must change folded R: tight={:?} wide={:?}",
+        tight_m.expectancy_r,
+        wide.expectancy_r,
+    );
+```
+Update the test's doc-comment "tight VolStop" → "tight FixedStop". (The vol stop's
+varying-distance behaviour is covered by `vol_stop_composite.rs`, Task 4.)
+
+- [ ] **Step 2: Delete `VolStop` from `stop_rule.rs`**
 
 Remove the entire `pub struct VolStop`, its `impl VolStop`, its `impl Node for VolStop`,
 and the two inline tests `vol_stop_is_none_until_warm` and
@@ -235,18 +268,17 @@ the module doc-comment: the volatility stop is now a composition (see
 `crates/aura-engine/tests/vol_stop_composite.rs`), `FixedStop` the only stop-rule
 primitive.
 
-- [ ] **Step 2: Drop the `VolStop` re-export**
+- [ ] **Step 3: Drop the `VolStop` re-export**
 
 In `crates/aura-std/src/lib.rs`: change `pub use stop_rule::{FixedStop, VolStop};` to
 `pub use stop_rule::FixedStop;`.
 
-- [ ] **Step 3: Build the workspace green-unchanged**
+- [ ] **Step 4: Build the workspace green**
 
-Run: `cargo build --workspace 2>&1 | grep -E "error|cannot find VolStop" | head`
-Expected: empty (no code outside `stop_rule.rs`/`lib.rs` references `VolStop` — the iter-1
-E2E uses `FixedStop`). Then `cargo test -p aura-std stop_rule` Expected: PASS
-(`fixed_stop_is_constant_after_first_price`). Then `cargo test --workspace 2>&1 | tail -5`
-Expected: all green.
+Run: `cargo build --workspace 2>&1 | grep -E "error" | head`
+Expected: empty. Then `cargo test --workspace 2>&1 | tail -8`
+Expected: all green (incl. the migrated
+`r_is_stop_defined_two_stops_fold_to_different_expectancy`).
 
 ---
 
@@ -345,9 +377,10 @@ Run: `cargo clippy --workspace --all-targets -- -D warnings` Expected: clean.
   Task 4 match the builders the implementer verifies.
 - **Step granularity:** each step 2-5 min; Tasks 1-2 are patterned mirrors of `sub.rs`.
 - **No commit steps:** none.
-- **Compile-gate ordering:** Task 3's `VolStop` removal threads its only references
-  (`stop_rule.rs` + the `lib.rs` re-export) within Task 3; the workspace build gate is
-  satisfiable because no other code references `VolStop` (iter-1 E2E uses `FixedStop`).
+- **Compile-gate ordering:** `VolStop` IS referenced by `stage1_r_e2e.rs`
+  (`VolStop::new(1, 1.0)`), so Task 3 threads all three references — `stop_rule.rs`, the
+  `lib.rs` re-export, AND the E2E caller (migrated to a tight `FixedStop` in Step 1, BEFORE
+  the deletion) — within Task 3; the workspace build gate at Step 4 is then satisfiable.
 - **Verification filters resolve:** `cargo test -p aura-std mul` / `sqrt` /
   `stop_rule` / `-p aura-engine --test vol_stop_composite` match the new/kept named
   tests; the removal gate uses `cargo build --workspace` + the unfiltered suite.