feat(std): meaning lines for all 27 shipped node schemas

C29 compile/unit seam, task 3 of the self-description plan: every
aura-std NodeSchema literal threads its one-line doc. Four texts were
corrected against the actual eval/finalize semantics rather than taken
from the plan table verbatim: Delay is a lag-N register (not one-step),
GatedRecorder flushes an ungated final row at finalize, Latch is a
level-sensitive set/reset register (captures no input), SeriesReducer
emits its single summary row at finalize (not per cycle).

Gate: cargo build -p aura-std --lib clean; full std test run follows at
the all-crates gate once strategy/backtest/engine thread their sites
(the earlier per-crate test gate was unsatisfiable in isolation --
std's dev-deps pull crates whose sites belong to later tasks).

refs #316

crates/aura-std/src/select.rs

@@ -38,6 +38,7 @@ impl Select {
                 ],
                 output: vec![FieldSpec { name: "value".into(), kind: ScalarKind::F64 }],
                 params: vec![],
+                doc: "chooses between two inputs by a boolean selector",
             },
             |_| Box::new(Select::new()),
         )