fix(skm): accept floating-point population (JSON Struct inputs)

JSON inputs arriving via google.protobuf.Struct (the gRPC submit
path, HubClient.submit jsonInputs) are always floating-point, so a
u64 population field rejected `180000.0` with an INVALID_ARGUMENT.
Make population f64 — a count is exact in f64 up to 2^53, the SKM
product is f64 anyway, and Display still renders whole numbers
without a decimal. Unblocks live skm scoring from the Recl∆Im app.

Signed-off-by: flemming-it <sf@flemming.it>

src/lib.rs

@@ -22,7 +22,12 @@ struct InputDuties {
 
 #[derive(Debug, Deserialize)]
 struct InputDuty {
-    population: u64,
+    // f64, not u64: JSON inputs arriving via google.protobuf.Struct
+    // (HubClient.submit jsonInputs) are always floating-point, so a
+    // u64 field rejects `180000.0`. A population is a count, but f64
+    // represents it exactly up to 2^53 and the SKM product is f64
+    // anyway. Display still renders whole numbers without a decimal.
+    population: f64,
     frequency: f32,
     tariff_eur_per_hour: f32,
     time_hours_per_case: f32,
@@ -59,7 +64,7 @@ pub fn invoke(_ctx: Context, inputs: Inputs) -> Result<Outputs, ModuleError> {
         if d.frequency < 0.0 || d.tariff_eur_per_hour < 0.0 || d.time_hours_per_case < 0.0 {
             return Err(ModuleError::invalid_input("negative P/F/T/h not allowed"));
         }
-        let value = d.population as f64
+        let value = d.population
             * d.frequency as f64
             * d.tariff_eur_per_hour as f64
             * d.time_hours_per_case as f64;