unicode : include '~' in collapsed symbol class (#26972)

The collapsed \p{S} class was missing '~', which split " ~" into
separate pre-tokens and prevented the Ġ~ BPE merge used by DeepSeek V4.
This caused re-tokenized prompts to diverge from sampled tokens and
broke KV cache reuse.

Assisted-by: Codex

tests/test-unicode.cpp

@@ -0,0 +1,24 @@
+#include "../src/unicode.h"
+
+#include <cstdio>
+#include <string>
+#include <vector>
+
+int main() {
+    const std::vector<std::string> regex_exprs = {
+        "[~][A-Za-z]+| ?[\\p{S}]+|\\s+",
+    };
+    const std::vector<std::string> expected = { " ~", "foo" };
+    const auto actual = unicode_regex_split(" ~foo", regex_exprs, false);
+
+    if (actual != expected) {
+        fprintf(stderr, "unexpected split:");
+        for (const auto & piece : actual) {
+            fprintf(stderr, " [%s]", piece.c_str());
+        }
+        fprintf(stderr, "\n");
+        return 1;
+    }
+
+    return 0;
+}