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}"
     );
 }