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.

dsl/lexer.go

@@ -371,8 +371,6 @@ func (l *Lexer) readNumberOrDuration(line, col int) (Token, error) {
 	return Token{TOKEN_INT, buf.String(), line, col}, nil
 }
 
-// tryReadSizeSuffix reads memory size suffixes: B, K, KB, KiB, M, MB, MiB, G, GB, GiB.
-// Units are case-sensitive uppercase to avoid collision with duration "m" (minutes).
 func (l *Lexer) tryReadSizeSuffix() string {
 	ch, ok := l.peek()
 	if !ok {