feat(engine): FlatTap type + FlatGraph.taps field (#282)

Second slice: the resolved-tap carrier. FlatTap { name, node, field } is the
output-side twin of SourceSpec — its name survives compile, load-bearing for
by-name binding (#275). FlatGraph gains a taps field, threaded (empty) through
every construction site the compiler enumerates. Bootstrap ignores it (taps
materialize as real recorder nodes/edges before bootstrap in a later slice).
Purely additive: the inert-producer pin and full suite stay green.

refs #282

crates/aura-ingest/examples/shared/breakout_real.rs

@@ -187,7 +187,7 @@ pub fn build_harness() -> (Harness, Taps) {
         .map(|slot| SourceSpec::raw(ScalarKind::F64, vec![Target { node: RESAMPLE, slot }]))
         .collect();
 
-    let h = Harness::bootstrap(FlatGraph { nodes, signatures, sources, edges })
+    let h = Harness::bootstrap(FlatGraph { nodes, signatures, sources, edges, taps: Vec::new() })
         .expect("valid breakout DAG");
     (h, Taps { equity: rx_equity, held: rx_held, bars: rx_bars, breakout: rx_breakout })
 }

crates/aura-ingest/tests/real_bars.rs

@@ -60,6 +60,7 @@ fn run_sample_over(prices: Vec<(Timestamp, Scalar)>) -> RunReport {
             Edge { from: 4, to: 5, slot: 0, from_field: 0 }, // equity -> sink 5
             Edge { from: 3, to: 6, slot: 0, from_field: 0 }, // exposure -> sink 6
         ],
+        taps: Vec::new(),
     })
     .expect("valid signal-quality DAG");
 

crates/aura-ingest/tests/streaming_seam.rs

@@ -66,6 +66,7 @@ fn run_sample(window: (Timestamp, Timestamp), source: Box<dyn Source>) -> RunRep
             Edge { from: 4, to: 5, slot: 0, from_field: 0 },
             Edge { from: 3, to: 6, slot: 0, from_field: 0 },
         ],
+        taps: Vec::new(),
     })
     .expect("valid signal-quality DAG");