Improve Zstd.decompress performance by avoiding unnecessary memory copy

Currently, `rb_decompress` allocates a temporary buffer and copies the input string using `ALLOC_N` and `memcpy`.
This overhead is significant, especially for large data, and doubles the memory usage during decompression.

## benchmark
```ruby
require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'zstd-ruby'
  gem 'benchmark-ips'
end

require 'securerandom'
large_data = SecureRandom.random_bytes(1024 * 1024 * 100)
compressed_data = Zstd.compress(large_data)

Benchmark.ips do |x|
  x.time = 15

  x.report("zstd decompress") do
    Zstd.decompress(compressed_data)
  end
end
```

## before
```
$ ruby zstd.rb
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [x86_64-linux]
Warming up --------------------------------------
     zstd decompress     1.000 i/100ms
Calculating -------------------------------------
     zstd decompress     11.148 (± 9.0%) i/s   (89.70 ms/i) -    167.000 in  15.085672s
```

## after
```
$ ruby zstd.rb
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [x86_64-linux]
Warming up --------------------------------------
     zstd decompress     2.000 i/100ms
Calculating -------------------------------------
     zstd decompress     20.890 (± 9.6%) i/s   (47.87 ms/i) -    312.000 in  15.015814s
```

Author: Watson1978

ext/zstdruby/zstdruby.c

@@ -78,9 +78,7 @@ static VALUE rb_decompress(int argc, VALUE *argv, VALUE self)
   StringValue(input_value);
 
   size_t in_size = RSTRING_LEN(input_value);
-  const unsigned char *in_r = (const unsigned char *)RSTRING_PTR(input_value);
-  unsigned char *in = ALLOC_N(unsigned char, in_size);
-  memcpy(in, in_r, in_size);
+  const unsigned char *in = (const unsigned char *)RSTRING_PTR(input_value);
 
   size_t off = 0;
   const uint32_t ZSTD_MAGIC = 0xFD2FB528U;
@@ -107,22 +105,19 @@ static VALUE rb_decompress(int argc, VALUE *argv, VALUE self)
     if (magic == ZSTD_MAGIC) {
       ZSTD_DCtx *dctx = ZSTD_createDCtx();
       if (!dctx) {
-        xfree(in);
         rb_raise(rb_eRuntimeError, "ZSTD_createDCtx failed");
       }
 
       VALUE out = decode_one_frame(dctx, in + off, in_size - off, kwargs);
 
       ZSTD_freeDCtx(dctx);
-      xfree(in);
       RB_GC_GUARD(input_value);
       return out;
     }
 
     off += 1;
   }
 
-  xfree(in);
   RB_GC_GUARD(input_value);
   rb_raise(rb_eRuntimeError, "not a zstd frame (magic not found)");
 }