Implement Parameters API

- Add `.name()` builder method on all 5 parameter types for custom labels
- Add `CompletedTrial::get(&param)` for typed parameter access
- Add `Display` impl on `ParamValue`
- Add prelude module at `optimizer::prelude::*`
- Shadow `_with_sampler` methods on `Study<f64>` so `optimize()` auto-uses
  the configured sampler; deprecate `_with_sampler` variants
- Use runtime `Any` downcasting with `trial_factory` to avoid E0592
- Update all examples and tests to use the new API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/param.rs

@@ -14,3 +14,13 @@ pub enum ParamValue {
     /// A categorical parameter value, stored as an index into the choices array.
     Categorical(usize),
 }
+
+impl core::fmt::Display for ParamValue {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Float(v) => write!(f, "{v}"),
+            Self::Int(v) => write!(f, "{v}"),
+            Self::Categorical(v) => write!(f, "category({v})"),
+        }
+    }
+}