feat(std): Sign and Select cells — the Bool→F64 bridge pair

First half of the #281 clock-enable set: Sign promotes the private
sign0() semantics to vocabulary (strict comparisons; NaN and -0.0
fall to 0.0), Select is the stateless 2:1 mux that consumes Bool
back into the F64 plane. Both zero-arg, Firing::Any, rostered
(count pins 29→31); an engine E2E drives both through the real
GraphBuilder→compile→run seam.

refs #281

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(),
-        29,
-        "the std-only (no project) vocabulary has exactly the roster's 29 entries: {stdout}"
+        31,
+        "the std-only (no project) vocabulary has exactly the roster's 31 entries: {stdout}"
     );
 }