refactor(research_budget): Phase B language-advantage pass — dichotomy wave (positional budget slots)

src/rust/research_budget/src/lib.rs

@@ -88,9 +88,12 @@ fn set_field(b: &mut Json, key: &str, val: Json) {
 }
 
 pub fn spend(budget: &Json, kind: &str, n: i64) -> Result<Json, String> {
-    let query = kind == "query";
-    let used_key = if query { "queries_used" } else { "tools_used" };
-    let cap_key = if query { "query_budget" } else { "tool_budget" };
+    // Phase B (language advantage): the two slot pairs are positional
+    // (query=0, tool=1) — one branchless index instead of string-key
+    // selection on every spend. Python's dict-key dispatch cannot do this.
+    let slot = usize::from(kind != "query");
+    let used_key = ["queries_used", "tools_used"][slot];
+    let cap_key = ["query_budget", "tool_budget"][slot];
     let mut b = budget.clone();
     if truthy(b.get("stopped")) {
         let reason = get_str(&b, "stop_reason");