refactor: modernize stdlib usage and move matchers into dsl

- Move Matcher types and matching logic from runner/matcher.go into
  the dsl package as methods on the Matcher types. Runner now calls
  t.Stdout.Match(label, actual) instead of type-switching via a
  package-level applyMatcher helper.
- Replace custom contains/containsString helpers with slices.Contains
  in dsl/build.go and runner/compiler.go.
- Use maps.Copy instead of manual map copy in BuildConfig.MergeFrom.
- Adopt strings.SplitSeq, strings.CutPrefix and the `for range N` loop
  form in runner/limiter_linux.go.
- Ignore example/imdb build artifact.

runner/runner.go

@@ -558,10 +558,10 @@ func (r *Runner) runTest(t *dsl.Test) *TestResult {
 		tr.addFailure("exit code: expected %d, got %d", *t.ExitCode, actualCode)
 	}
 
-	for _, f := range applyMatcher("stdout", t.Stdout, tr.ActualStdout) {
+	for _, f := range t.Stdout.Match("stdout", tr.ActualStdout) {
 		tr.addFailure("%s", f)
 	}
-	for _, f := range applyMatcher("stderr", t.Stderr, tr.ActualStderr) {
+	for _, f := range t.Stderr.Match("stderr", tr.ActualStderr) {
 		tr.addFailure("%s", f)
 	}