feat(aura-engine): composite output is a named multi-field record

Replace a composite's single output port (OutPort { node, field }) with
an ordered, named record (Vec<OutField { node, field, name }>). A
composite can now re-export K interior fields as one output; a consumer
selects one via the existing Edge::from_field — the same field-wise
selector that already works for multi-output leaf producers (OHLCV).

Why: multi-line indicators (MACD, Bollinger, Stochastic, Ichimoku) could
not be authored as a composition and re-exported as a unit — only one
line escaped the boundary. The MACD PoC was forced to expose only the
histogram. This lifts the three `field == 0` / `from_field == 0` caps at
the composite boundary (inline_composite nested arm, rewrite_edge
composite arm), range-checking the index instead.

Boundary completion, not an invariant change:
- C8/C7/C4 untouched — one port, one row per eval, K base columns (a
  3-line MACD is identical in kind to OHLCV).
- C23 honoured — re-export names live at the blueprint boundary only and
  are dropped in the compilat (raw index wiring). compiled_view_golden
  stayed byte-identical (the regression guard): no name leaked into the
  flat graph.

The MACD PoC now re-exports macd / signal / histogram as a record; the
strategy still trades the histogram, selected via from_field: 2. The
render shows K [out:<name>] markers per multi-output composite (input
roles stay [in:<index>] — naming them is the dependent #41).

Output naming ships with the capability (OutField is born name-ready) so
#41 — composite param aliasing + input-role naming — does not reopen the
type.

Verification: cargo build/test/clippy --workspace -- -D warnings all
green; aura-engine 62 tests (+3 capability, +1 through-run E2E), aura-cli
unchanged-count green; compiled_view_golden byte-identical.

Known debt (out of scope, pre-existing): the non-workspace construction-
layer fieldtests (fieldtests/milestone-construction-layer/mc_1..mc_4)
carry their OutField sweep but still do not compile standalone — they use
the Sma::new / BlueprintNode::from(Sma) API retired in the cycle-0016
value-empty migration, never propagated to these fixtures. Tracked as a
follow-up.

closes #40

crates/aura-cli/src/graph.rs

@@ -100,8 +100,8 @@ fn collect_distinct_composites(bp: &Blueprint) -> Vec<&Composite> {
 
 /// Render one composite's interior as a flat graph: interior leaves as `[type]`,
 /// nested composites as opaque `[name]`, plus an `[in:k]` entry marker per input
-/// role (wired to its interior targets) and an `[out]` marker (wired from the
-/// output port). Prefixed `"<name>:\n"`.
+/// role (wired to its interior targets) and an `[out:<name>]` marker per
+/// re-exported output field (wired from its producer). Prefixed `"<name>:\n"`.
 fn render_definition(c: &Composite) -> String {
     let mut labels: Vec<String> = Vec::with_capacity(c.nodes().len());
     for inner in c.nodes() {
@@ -121,9 +121,11 @@ fn render_definition(c: &Composite) -> String {
             edges.push((in_id, t.node));
         }
     }
-    let out_id = labels.len();
-    labels.push("out".to_string());
-    edges.push((c.output().node, out_id));
+    for of in c.output() {
+        let out_id = labels.len();
+        labels.push(format!("out:{}", of.name));
+        edges.push((of.node, out_id));
+    }
 
     format!("{}:\n{}", c.name(), render_flat(&labels, &edges))
 }