core: add test-name filters to the vtest harness

run_all() takes an optional substring list; vtest_main forwards argv and a
comma-separated $VT_ONLY. No filter => run everything, as before. Makes
iterating on one slow end-to-end case (the engine suite) practical
without a framework swap.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01HPPSGhiArbvQgwC2DNiURS

core/tests/support/vtest.hpp

@@ -81,9 +81,17 @@ std::string show(const T &v) {
     }
 }
 
-inline int run_all() {
+inline int run_all(const std::vector<std::string> &filters = {}) {
     int failed_cases = 0;
     for (const auto &c : registry()) {
+        if (!filters.empty()) {
+            bool match = false;
+            for (const auto &f : filters)
+                if (std::string_view(c.name).find(f) != std::string_view::npos)
+                    match = true;
+            if (!match)
+                continue;
+        }
         int before = stats().failures;
         stats().current_fatal = false;
         std::fprintf(stderr, "[ RUN  ] %s\n", c.name);

core/tests/support/vtest_main.cpp

@@ -1,6 +1,29 @@
 // vtest_main.cpp — shared entry point for every CORE test binary.
+#include <cstdlib>
+#include <string>
+#include <vector>
+
 #include "vtest.hpp"
 
-int main() {
-    return ::vt::run_all();
+// Optional filters: each argv argument (or a comma-separated entry in $VT_ONLY) is a
+// substring; a test runs only if its name contains one of them. No filters => run all.
+int main(int argc, char **argv) {
+    std::vector<std::string> filters;
+    for (int i = 1; i < argc; ++i)
+        filters.emplace_back(argv[i]);
+    if (const char *env = std::getenv("VT_ONLY")) {
+        std::string cur;
+        for (const char *p = env;; ++p) {
+            if (*p == ',' || *p == '\0') {
+                if (!cur.empty())
+                    filters.push_back(cur);
+                cur.clear();
+                if (*p == '\0')
+                    break;
+            } else {
+                cur.push_back(*p);
+            }
+        }
+    }
+    return ::vt::run_all(filters);
 }