fix(group): force leading slash on Go route paths for contract-id parity

normalizeHttpPath (the shared contract-id normalizer) does not add a
leading slash, while ingestion's normalizeExtractedRoutePath always
does — so a literal path "x" or a slashless Group("api") prefix split
the contract id across the two strategies (`http::GET::x` vs
`http::GET::/x`). After joining and collapsing "//", ensure the result
starts with "/" so both strategies feed the same bytes into the id.
Gin also panics when registering a route path without a leading slash.

Tests: go-gin-route-groups 35, group suite 1298 (all green).

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

gitnexus/src/core/group/extractors/http-patterns/go.ts

@@ -108,7 +108,13 @@ function joinRoutePath(prefix: string, relative: string): string {
   // just the join — because ingestion's normalizeExtractedRoutePath collapses
   // all "//" while the downstream contract-id normalizer does not: a path
   // that keeps "//" would split into two contract ids across the strategies.
-  return joined.replace(/\/+/g, '/');
+  const collapsed = joined.replace(/\/+/g, '/');
+  // Force a leading "/" for the same reason: ingestion's
+  // normalizeExtractedRoutePath always adds one, while normalizeHttpPath (the
+  // shared contract-id normalizer) does not — a literal "x" or a slashless
+  // Group("api") prefix would emit `...::x` here and `...::/x` there. Gin
+  // also refuses a registration path that does not start with "/".
+  return collapsed.startsWith('/') ? collapsed : `/${collapsed}`;
 }
 
 /** `parent.Group("/p", mw...)` → its receiver and literal prefix; null otherwise. */

gitnexus/test/unit/group/go-gin-route-groups.test.ts

@@ -441,6 +441,28 @@ func routes(r *gin.Engine) {
     ]);
   });
 
+  it('forces a leading slash on slashless literals and group prefixes', () => {
+    // Ingestion's normalizeExtractedRoutePath always adds a leading "/" but
+    // normalizeHttpPath (the shared contract-id normalizer) does not — a path
+    // like "x" would become `http::GET::x` here and `http::GET::/x` there,
+    // splitting the id across the two strategies. Gin likewise panics when a
+    // route is registered without one.
+    expect(
+      providers(`package main
+func routes(r *gin.Engine) {
+	r.GET("x", h.X)
+	g := r.Group("api")
+	g.GET("y", h.Y)
+	r.GET("", h.Root)
+}
+`),
+    ).toEqual([
+      { method: 'GET', path: '/x', name: 'X' },
+      { method: 'GET', path: '/api/y', name: 'Y' },
+      { method: 'GET', path: '/', name: 'Root' },
+    ]);
+  });
+
   it("binds echo's handler (first argument) when the file imports echo only", () => {
     // echo is `GET(path, handler, middleware...)` — the handler is second,
     // not last, so a middleware selector must not become the route's name.