test: run filtered tests in one process by default

Someone filtering is usually debugging one case and wants as little
variability as possible. TEST_JOBS still overrides it.

AGENTS.md

@@ -15,7 +15,7 @@ TEST_FILTER="WebApi: #selector_all" make test   # Filter main + subtest (separat
 TEST_VERBOSE=true make test
 TEST_FAIL_FIRST=true make test
 METRICS=true make test                          # Capture allocation/duration metrics as JSON
-TEST_JOBS=1 make test                           # Run in one process (default: up to 4 in parallel)
+TEST_JOBS=1 make test                           # Run in one process (default: up to 4, or 1 when filtered)
 ```
 
 The custom test runner (`src/test_runner.zig`) detects memory leaks in debug builds. **A test that allocates without freeing fails** — not just lints.

README.md

@@ -350,7 +350,7 @@ TEST_FILTER="WebApi: #selector_all" make test   # Filter main + subtest (separat
 TEST_VERBOSE=true make test
 TEST_FAIL_FIRST=true make test
 METRICS=true make test                          # Capture allocation/duration metrics as JSON
-TEST_JOBS=1 make test                           # Run in one process (default: up to 4 in parallel)
+TEST_JOBS=1 make test                           # Run in one process (default: up to 4, or 1 when filtered)
 ```
 
 ### End to end tests

src/test_runner.zig

@@ -568,13 +568,18 @@ const Env = struct {
             .verbose = readEnvBool(map, "TEST_VERBOSE", false),
             .fail_first = readEnvBool(map, "TEST_FAIL_FIRST", false),
             // metrics are compared across runs, so keep them to one process
-            .jobs = if (metrics) 1 else readEnvInt(map, "TEST_JOBS") orelse defaultJobs(),
+            .jobs = if (metrics) 1 else readEnvInt(map, "TEST_JOBS") orelse defaultJobs(filter, subfilter),
             .claim = readEnv(map, "TEST_CLAIM"),
             .report = readEnv(map, "TEST_REPORT"),
         };
     }
 
-    fn defaultJobs() usize {
+    /// A filtered run is usually someone debugging one case, who wants as
+    /// little variability as possible, so it stays in one process unless asked.
+    fn defaultJobs(filter: ?[]const u8, subfilter: ?[]const u8) usize {
+        if (filter != null or subfilter != null) {
+            return 1;
+        }
         const cpus = std.Thread.getCpuCount() catch 1;
         return @min(cpus, 4);
     }