refactor(aura-ingest): fuse drain_trace taps via join_on_ts; doc the cadence rule

drain_trace now joins its four recording-sink taps through aura-engine's
join_on_ts instead of a hand-rolled per-tap HashMap, resolving the #93
zip-by-index panic at its source: the four taps fire at different
cardinalities (breakout one bar short via cold Delay(1); bars_since_open
filtered by Session), so a positional zip misaligns or panics. The BarTrace
mapping stays a thin layer with its caller-side defaults unchanged
(held->0.0, bars_since_open->-1, breakout->false) — the engine reports
presence, the consumer interprets absence. Behaviour is byte-preserved: the
gated GER40 tests (ger40_breakout_real, ger40_breakout_blueprint,
open_ohlc_seam) stay green against the real archive.

The Recorder doc gains a prose note on the multi-tap cadence rule (join on
ts, never by index), pointing at join_on_ts by name rather than rustdoc link
since aura-std does not depend on aura-engine.

Verified: cargo build/test/clippy --workspace --all-targets clean; the
_by_ts hand-rolled join is gone (acceptance grep empty).

closes #93

crates/aura-std/src/recorder.rs

@@ -14,6 +14,13 @@ use std::sync::mpsc::Sender;
 /// the newest value of every column and sends the row to `tx`; it returns `None`
 /// (records, forwards nothing) and `None` during warm-up until all columns have a
 /// value.
+///
+/// Multiple taps fire at their own node's cadence, so their recorded streams have
+/// different lengths and different firing instants. To fuse several taps into one
+/// trace, join on the recorded timestamp — never by positional index, which
+/// misaligns the columns or panics. The engine's report layer ships a `join_on_ts`
+/// helper (in `aura-engine`) for exactly this; it is named in prose rather than
+/// linked because `aura-std` does not depend on `aura-engine`.
 pub struct Recorder {
     kinds: Vec<ScalarKind>,
     tx: Sender<(Timestamp, Vec<Scalar>)>,