feat(aura): render composite output re-exports as producer bindings

In the blueprint view's `where:` section, a composite's output re-exports
now fold onto their producing node's label as a binding
(`[macd := Sub(#Ef,#Es)]`) instead of each becoming a standalone terminal
node wired from its producer. An OutField re-exports an existing interior
node's value — it is never a new node — so the old form drew false
terminals (MACD's macd line feeds the signal EMA and histogram internally
yet rendered as a dead-end leaf) and inflated the node count by the output
arity (MACD where: 8 -> 6 nodes; sma_cross drops its [cross] stub + edge).
The drawn graph is now exactly the computation DAG; the `:=` prefix is the
visual signal that a node's value escapes the boundary. Input roles stay
standalone entry nodes (no producer to annotate) — the asymmetry is
deliberate.

render_definition drops its per-OutField node+edge loop; a new
output_binding helper yields the `name := ` / tuple `(a, b) := ` prefix.
Pure render layer: C8/C23 untouched, OutField.name never reaches the
compilat, compiled_view_golden byte-identical, MACD run deterministic and
unchanged in metrics.

Test blast radius was wider than spec 0022 §Components / the plan's Scope
note counted: besides the MACD pinning test, four more sites assert a
producer that carries an OutField and so gain the `:=` prefix —
blueprint_view_defines_each_composite_once, the two fan-in identifier
tests, the cli_run integration test, and (missed by the plan, caught at
implement) the full-render blueprint_view_golden. All re-pins are forced
by the design, zero judgement. The output_binding tuple arm is unexercised
by current fixtures (no composite re-exports >1 field from one node).

Verified: cargo test --workspace 0 failed; clippy --all-targets
-D warnings clean.

closes #46

crates/aura-cli/tests/cli_run.rs

@@ -174,8 +174,8 @@ fn graph_renders_source_derived_fan_in_identifiers() {
     assert_eq!(out.status.code(), Some(0), "`aura graph` exit: {:?}", out.status);
     let stdout = String::from_utf8(out.stdout).expect("utf-8 stdout");
     assert!(
-        stdout.contains("[Sub(#Sf,#Ss)]"),
-        "fan-in inputs must be source-derived (#Sf/#Ss), not positional: {stdout}"
+        stdout.contains("[cross := Sub(#Sf,#Ss)]"),
+        "fan-in inputs source-derived (#Sf/#Ss), bound as output `cross`: {stdout}"
     );
     // the retired positional form must not reappear.
     assert!(