perf: Cache Spec.basePath at construction (W1)

basePath() was previously recomputed via URI.create on every request
through RequestPreparationFilter.stripBasePath. The result is
spec-static, so promote it to a record component populated in
Spec.from(...). Eliminates ~100 MB of URI allocations and ~26 CPU
samples per JFR run; k6 throughput +2.6% (44.2k -> 45.3k rps),
p95 -40 µs.

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

Author: thced

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

@@ -16,7 +16,8 @@ public record Spec(
     List<Server> servers,
     List<Operation> operations,
     Map<String, Schema> componentSchemas,
-    Map<String, Parameter> componentParameters) {
+    Map<String, Parameter> componentParameters,
+    String basePath) {
 
   private static final String SCHEMA_KEY = "schema";
 
@@ -32,14 +33,21 @@ public static Spec from(Map<String, Object> raw) {
     List<Operation> operations =
         parseOperations(
             (Map<String, Object>) raw.getOrDefault("paths", Map.of()), componentParameters);
-    return new Spec(openapi, info, servers, operations, componentSchemas, componentParameters);
+    return new Spec(
+        openapi,
+        info,
+        servers,
+        operations,
+        componentSchemas,
+        componentParameters,
+        computeBasePath(servers));
   }
 
-  public String basePath() {
+  private static String computeBasePath(List<Server> servers) {
     if (servers.isEmpty()) {
       throw new IllegalStateException("no servers declared");
     }
-    return Optional.ofNullable(URI.create(servers.get(0).url()).getPath()).orElse("");
+    return Optional.ofNullable(URI.create(servers.getFirst().url()).getPath()).orElse("");
   }
 
   public Schema resolveSchema(String ref) {

src/test/java/com/retailsvc/http/internal/RequestPreparationFilterTest.java

@@ -44,7 +44,13 @@ private HttpExchange exchange(String method, String path, byte[] body) {
 
   private Spec specWith(Operation... ops) {
     return new Spec(
-        "3.1.0", new Info("t", "1"), List.of(new Server("/")), List.of(ops), Map.of(), Map.of());
+        "3.1.0",
+        new Info("t", "1"),
+        List.of(new Server("/")),
+        List.of(ops),
+        Map.of(),
+        Map.of(),
+        "");
   }
 
   private Filter newFilter(Spec spec) {