feat(aura-core): PrimitiveBuilder::bind — fix a node param as a structural constant

Add `PrimitiveBuilder::bind(slot, value)`: bind a declared param to a structural
constant, removed from param_space entirely rather than pinned in the injected
vector. This closes the gap #55 names — param-bearing nodes (Sma/Exposure/LinComb)
had no constant path, only the no-sweep-mode SimBroker precedent. The motivating
case is "SMA2-entry": the 2 is bound to the two-candle construction, so sweeping
it would enumerate deformed strategies as valid family members.

Approach (Option B, builder-level — the originally-planned direction): bind shrinks
the builder's declared param surface (schema.params) and wraps its build closure to
re-splice the captured constant at its original positional slot. The construction
layer (collect_params / lower_items / param_space / compile_with_params) is
byte-unchanged: both dock sites already key off builder.params(), so the shrink
propagates for free. Addressing is by param name (the by-name authoring address
space, C23/0032); the compilat stays wired by raw index — names never reach it.

bind enforces "exactly one open param matches" itself (collect-all-then-reject,
panic on zero / on duplicate), mirroring the resolve binder's posture, because a
node's own schema.params is not covered by check_param_namespace_injective. Chained
binds reconstruct the correct positional vector because each layer computes its
slot index relative to the param list it sees (no global position table).

Considered and rejected: a per-node builder_const(N) constructor (explodes
combinatorially over which subset is constant, not dynamic); a ParamBinding overlay
struct on Composite (would need new bookkeeping the builder-level form avoids).

Verified: cargo build/clippy/test --workspace all green; new tests cover slot
removal, positional reconstruction (chained + partial, via eval), the three panic
paths, and the composite param_space projection; blueprint.rs construction layer
confirmed byte-unchanged.

Deferred (out of scope, follow-up issue): exporting a named frozen strategy as a
blueprint-as-values collection — the issue's third fork, needs its own brainstorm.

closes #55

crates/aura-engine/src/blueprint.rs

@@ -1882,6 +1882,30 @@ mod tests {
         assert_eq!(space[2].kind, ScalarKind::F64);
     }
 
+    #[test]
+    fn param_space_reflects_only_open_knobs() {
+        use aura_std::{Exposure, Sma};
+        // "sma2_entry": Sma "bias" with length bound to 2 (a structural constant)
+        // plus Exposure "exp" whose `scale` stays open. The bound knob must be
+        // absent from param_space; only the open one remains.
+        let strat = Composite::new(
+            "sma2_entry",
+            vec![
+                Sma::builder().named("bias").bind("length", Scalar::I64(2)).into(),
+                Exposure::builder().named("exp").into(),
+            ],
+            vec![], // edges — irrelevant to param_space()
+            vec![], // input_roles
+            vec![], // output
+        );
+        let space = strat.param_space();
+        let names: Vec<&str> = space.iter().map(|p| p.name.as_str()).collect();
+        // collect_params runs with an empty prefix, so a top-level leaf is qualified
+        // by its OWN node segment, not the root composite's name → "exp.scale".
+        // `bias.length` is GONE (bound), not present-but-fixed.
+        assert_eq!(names, ["exp.scale"]);
+    }
+
     /// E2E (issue #34): the C23/#31 mirror invariant *under composite nesting*.
     /// `param_space()` (via `collect_params`) duplicates `lower_items`' depth-
     /// first traversal rather than sharing it, so the two orders must stay in