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/const_node.rs

@@ -33,6 +33,7 @@ impl Const {
                 inputs: vec![PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: "clock".into() }],
                 output: vec![FieldSpec { name: "value".into(), kind: ScalarKind::F64 }],
                 params: vec![ParamSpec { name: "value".into(), kind: ScalarKind::F64 }],
+                doc: "constant-valued stream from a single param",
             },
             |p| Box::new(Const::new(p[0].f64())),
         )