feat(std,engine): SessionFrankfurt — the Session node enters the closed roster

A Frankfurt-baked zero-arg Session preset (open 09:00 Europe/Berlin as
literals) joins the std vocabulary as SessionFrankfurt, declaring
period_minutes: I64 (default 15) as its one scalar knob — the
single-knob roster pattern the cost nodes established. A data-only
blueprint can now reach a session/time-of-day stream (bars_since_open),
unblocking time-of-day conditioners for the confirm/refute research
model. Roster count pins bump 28 -> 29 in both conscious-act sites.

Fork decision on the issue: the preset variant over a param-vocabulary
extension — a timezone param would need a string kind, breaching the
four-scalar discipline; the preset keeps the vocabulary closed and is
the additive extension the roster-exclusion comment anticipates. The
4-arg Session::builder stays untouched; in_session/session_open_ts
stay deferred (#154).

closes #261

crates/aura-cli/tests/graph_construct.rs

@@ -121,8 +121,8 @@ fn graph_introspect_vocabulary_lists_exactly_the_closed_roster_count() {
     let lines: Vec<&str> = stdout.lines().filter(|l| !l.is_empty()).collect();
     assert_eq!(
         lines.len(),
-        28,
-        "the std-only (no project) vocabulary has exactly the roster's 28 entries: {stdout}"
+        29,
+        "the std-only (no project) vocabulary has exactly the roster's 29 entries: {stdout}"
     );
 }
 

crates/aura-std/src/lib.rs

@@ -88,7 +88,7 @@ pub use rolling_max::RollingMax;
 pub use rolling_min::RollingMin;
 pub use scale::Scale;
 pub use series_reducer::SeriesReducer;
-pub use session::Session;
+pub use session::{Session, SessionFrankfurt};
 pub use sim_broker::SimBroker;
 pub use sizer::Sizer;
 pub use sma::Sma;

crates/aura-std/src/session.rs

@@ -32,7 +32,9 @@
 //!
 //! **Cold guard.** If the trigger column is empty (not yet fired) → `None`.
 
-use aura_core::{Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, PortSpec, PrimitiveBuilder, ScalarKind};
+use aura_core::{
+    Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, ParamSpec, PortSpec, PrimitiveBuilder, ScalarKind,
+};
 use chrono::{TimeZone, Timelike};
 
 /// Frankfurt-session bar indexer. Holds the open offset (minutes past local
@@ -76,6 +78,33 @@ impl Session {
     }
 }
 
+/// The zero-arg `SessionFrankfurt` roster preset (#261): the Frankfurt open
+/// (09:00 `Europe/Berlin`) baked as a literal, so the closed `std_vocabulary`
+/// roster can reach a `Session` without a structural construction argument
+/// (`Session::builder` itself stays absent from the roster — #155 scope).
+/// The one remaining knob, `period_minutes` (i64, conventionally 15 — the
+/// resampler's bar width), is declared as the node's sole `ParamSpec` and
+/// bound through it via the single-knob `CarryCost`/`ConstantCost`/
+/// `VolSlippageCost` pattern. This is additive: `Session::builder` is
+/// untouched and stays the vehicle for other timezones/opens.
+pub struct SessionFrankfurt;
+
+impl SessionFrankfurt {
+    /// The param-generic recipe: one `period_minutes` I64 knob, the Frankfurt
+    /// open/timezone baked in the closure.
+    pub fn builder() -> PrimitiveBuilder {
+        PrimitiveBuilder::new(
+            "SessionFrankfurt",
+            NodeSchema {
+                inputs: vec![PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: "trigger".into() }],
+                output: vec![FieldSpec { name: "bars_since_open".into(), kind: ScalarKind::I64 }],
+                params: vec![ParamSpec { name: "period_minutes".into(), kind: ScalarKind::I64 }],
+            },
+            |p| Box::new(Session::new(9, 0, chrono_tz::Europe::Berlin, p[0].i64())),
+        )
+    }
+}
+
 impl Node for Session {
     fn lookbacks(&self) -> Vec<usize> {
         vec![1]

crates/aura-std/src/vocabulary.rs

@@ -20,7 +20,7 @@
 use crate::{
     Abs, Add, And, Bias, CarryCost, Const, ConstantCost, Delay, Div, Ema, EqConst, FixedStop, Gt,
     Latch, LongOnly, Max, Min, Mul, PositionManagement, Resample, RollingMax, RollingMin, Scale,
-    Sizer, Sma, Sqrt, Sub, VolSlippageCost,
+    SessionFrankfurt, Sizer, Sma, Sqrt, Sub, VolSlippageCost,
 };
 use aura_core::PrimitiveBuilder;
 
@@ -81,6 +81,7 @@ std_vocabulary_roster! {
     "RollingMax" => RollingMax,
     "RollingMin" => RollingMin,
     "Scale" => Scale,
+    "SessionFrankfurt" => SessionFrankfurt,
     "Sizer" => Sizer,
     "SMA" => Sma,
     "Sqrt" => Sqrt,
@@ -129,7 +130,7 @@ mod tests {
         assert!(!std_vocabulary_types().contains(&"LinComb")); // construction-arg node
         assert!(!std_vocabulary_types().contains(&"Recorder")); // sink
         assert!(!std_vocabulary_types().contains(&"nope"));
-        // count guard: pins the roster at exactly 28 entries
-        assert_eq!(std_vocabulary_types().len(), 28);
+        // count guard: pins the roster at exactly 29 entries
+        assert_eq!(std_vocabulary_types().len(), 29);
     }
 }