audit: cycle 0041 — drift-clean (Source ingestion seam)

Architect drift review over 8b330e3..HEAD (the Source-seam cycle #71 plus the
interim #67 optimize work and refactors). No semantic or contract drift: the
seam faithfully preserves C3 (ms→epoch-ns at the one ingestion boundary), C4
(tie-by-source-index, byte-for-byte), and C7 (field-per-source SoA); the full
suite is byte-identical green and the gated streaming tests pass on real data.

Regression gate: `cargo test --workspace` green (the project declares no
dedicated regression script; the suite is the gate).

Tidied stale prose the cycle left behind (no behaviour change):
  - docs/design/INDEX.md (C12): replaced the cycle-0011 "deliberate eager gap"
    status note with a cycle-0041 realization note — the producer `Source` seam
    + streaming `M1FieldSource` now deliver per-source O(one-chunk) streaming;
    precisely scopes what remains open (cross-*sim* Arc<[T]> window sharing,
    still unbuilt until the orchestration families #66/#68/#69 consume it).
  - aura-core/src/lib.rs: dropped the now-shipped "Source trait + data-server
    ingestion" from the module doc's "still to come" list (aura-engine's twin
    line was fixed in the cycle; aura-core's was missed).
  - aura-ingest/src/lib.rs: module header now documents both coexisting paths
    (eager load_m1_window/M1Columns vs lazy streaming M1FieldSource).
  - aura-registry/src/lib.rs: rank_by/optimize doc comments linked to the
    private `metric_cmp` (a public→private intra-doc-link warning from the #67
    commit); demoted to plain code spans, so `cargo doc --workspace` is now
    warning-clean.

crates/aura-core/src/lib.rs

@@ -24,8 +24,7 @@
 //!   access into each input (closing the cycle-0001 read-side gap on
 //!   [`AnyColumn`]).
 //!
-//! Still to come (subsequent cycles): the `Source` trait + data-server ingestion
-//! and source-native time normalization (C3/C11), the broker-independent
+//! Still to come (subsequent cycles): the broker-independent
 //! position-event output and downstream broker nodes (C10), and the run registry
 //! (C18/C22).