Optimize fbuffer_append_str_repeat

Helps with pretty printting performance:

```
== Encoding activitypub.json (52595 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     1.746k i/100ms
Calculating -------------------------------------
               after     17.481k (± 1.0%) i/s   (57.20 μs/i) -     89.046k in   5.094341s

Comparison:
              before:    16038.4 i/s
               after:    17481.1 i/s - 1.09x  faster

== Encoding citm_catalog.json (500298 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    60.000 i/100ms
Calculating -------------------------------------
               after    608.157 (± 2.3%) i/s    (1.64 ms/i) -      3.060k in   5.034238s

Comparison:
              before:      525.3 i/s
               after:      608.2 i/s - 1.16x  faster

== Encoding twitter.json (466906 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   160.000 i/100ms
Calculating -------------------------------------
               after      1.606k (± 0.5%) i/s  (622.70 μs/i) -      8.160k in   5.081406s

Comparison:
              before:     1410.3 i/s
               after:     1605.9 i/s - 1.14x  faster
```

Author: byroot

ext/json/ext/fbuffer/fbuffer.h

@@ -177,12 +177,17 @@ static inline void fbuffer_inc_capa(FBuffer *fb, unsigned long requested)
     }
 }
 
-static void fbuffer_append(FBuffer *fb, const char *newstr, unsigned long len)
+static inline void fbuffer_append_reserved(FBuffer *fb, const char *newstr, unsigned long len)
+{
+    MEMCPY(fb->ptr + fb->len, newstr, char, len);
+    fbuffer_consumed(fb, len);
+}
+
+static inline void fbuffer_append(FBuffer *fb, const char *newstr, unsigned long len)
 {
     if (len > 0) {
         fbuffer_inc_capa(fb, len);
-        MEMCPY(fb->ptr + fb->len, newstr, char, len);
-        fbuffer_consumed(fb, len);
+        fbuffer_append_reserved(fb, newstr, len);
     }
 }
 
@@ -210,13 +215,15 @@ static void fbuffer_append_str(FBuffer *fb, VALUE str)
 
 static void fbuffer_append_str_repeat(FBuffer *fb, VALUE str, size_t repeat)
 {
+    const char *newstr = StringValuePtr(str);
     unsigned long len = RSTRING_LEN(str);
 
-    size_t total = repeat * len;
-    fbuffer_inc_capa(fb, total);
-
+    fbuffer_inc_capa(fb, repeat * len);
     while (repeat) {
-        fbuffer_append_str(fb, str);
+#ifdef JSON_DEBUG
+        fb->requested = len;
+#endif
+        fbuffer_append_reserved(fb, newstr, len);
         repeat--;
     }
 }