feat(aura-engine): Source::resident_records — probe ring residency via the trait

The residency probe was an inherent method on the concrete M1FieldSource, so a Box<dyn Source> consumer could not read it without downcasting (the adjacent friction noted in #95). Hoist it onto the Source trait as resident_records(&self) -> Option<usize> with a default of None, mirroring bounds()'s Option-for-unknown idiom: None = "does not report" (the eager VecSource, which has no streaming ring), distinct from Some(0) = "reports, holds none" (e.g. exhausted).

M1FieldSource overrides it to return Some(chunk len). Callers updated (streaming_seam, the two deep-dive fieldtest bins). Tests: a hermetic VecSource default-is-None check, and a &dyn Source probe in the gated residency test proving M1FieldSource residency is readable without a downcast.

refs #95

crates/aura-engine/src/harness.rs

@@ -70,6 +70,18 @@ pub trait Source {
     /// open-ended. Cursor-independent: a property of the data extent, not the
     /// read position.
     fn bounds(&self) -> Option<(Timestamp, Timestamp)>;
+
+    /// Records this source holds resident right now — the streaming-ring probe
+    /// (#95). `None` (the default) means the source does not report a residency:
+    /// the eager [`VecSource`] materializes its whole stream and has no bounded
+    /// ring. A lazy chunk source overrides this to return `Some(len)` of the chunk
+    /// it currently borrows, so a `Box<dyn Source>` consumer can budget memory
+    /// without downcasting. Mirrors [`bounds`](Source::bounds)' Option idiom:
+    /// `None` = "not reported," distinct from `Some(0)` = "reports, holds none"
+    /// (e.g. exhausted). Probes the aura source ring only, not whole-process RSS.
+    fn resident_records(&self) -> Option<usize> {
+        None
+    }
 }
 
 /// A `Source` over a fully materialized stream — the cycle-0011 eager shape,
@@ -493,6 +505,16 @@ mod tests {
         points.iter().map(|&(t, v)| (Timestamp(t), Scalar::f64(v))).collect()
     }
 
+    #[test]
+    fn vec_source_reports_no_residency_by_default() {
+        // The eager VecSource has no streaming ring, so it takes the Source trait
+        // default: residency is "not reported" (None), distinct from Some(0). The
+        // probe is reachable through `&dyn Source` without downcasting (#95).
+        let v = VecSource::new(f64_stream(&[(1, 1.0), (2, 2.0)]));
+        let probe: &dyn Source = &v;
+        assert_eq!(probe.resident_records(), None);
+    }
+
     #[test]
     fn seeded_source_same_seed_same_stream() {
         // Same seed -> bit-identical stream (C1/C12 determinism).