perf: Precompute Parameter JSON-pointer at parse time (W5)

The validation pointer "/<in>/<name>" was rebuilt with StringBuilder
on every request per parameter, even though it's spec-static. Promote
it to a record component computed once in the convenience constructor
and read it in RequestPreparationFilter.validateParameters.

k6 throughput +4.6% vs baseline (44.2k -> 46.2k rps), p95 -83 us.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thced

src/main/java/com/retailsvc/http/internal/RequestPreparationFilter.java

@@ -102,7 +102,7 @@ private void validateParameters(
       HttpExchange exchange, Operation op, Map<String, String> pathParams) {
     Map<String, String> query = null;
     for (Parameter p : op.parameters()) {
-      String pointer = "/" + p.in().name().toLowerCase(Locale.ROOT) + "/" + p.name();
+      String pointer = p.pointer();
       if (p.in() == Parameter.Location.QUERY && query == null) {
         query = parseQuery(exchange.getRequestURI().getQuery());
       }

src/main/java/com/retailsvc/http/spec/Parameter.java

@@ -1,8 +1,14 @@
 package com.retailsvc.http.spec;
 
 import com.retailsvc.http.spec.schema.Schema;
+import java.util.Locale;
+
+public record Parameter(String name, Location in, boolean required, Schema schema, String pointer) {
+
+  public Parameter(String name, Location in, boolean required, Schema schema) {
+    this(name, in, required, schema, "/" + in.name().toLowerCase(Locale.ROOT) + "/" + name);
+  }
 
-public record Parameter(String name, Location in, boolean required, Schema schema) {
   public enum Location {
     PATH,
     QUERY,