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/ast.go

@@ -1,6 +1,8 @@
 package dsl
 
-import "time"
+import (
+	"time"
+)
 
 type File struct {
 	Build        string
@@ -74,42 +76,3 @@ type Test struct {
 	Stderr   Matcher
 	OutFiles map[string]string
 }
-
-type Matcher interface {
-	matcherNode()
-}
-
-type ExactMatcher struct {
-	Value string
-}
-
-func (ExactMatcher) matcherNode() {}
-
-type ContainsMatcher struct {
-	Substr string
-}
-
-func (ContainsMatcher) matcherNode() {}
-
-type RegexMatcher struct {
-	Pattern string
-}
-
-func (RegexMatcher) matcherNode() {}
-
-type NumericEpsMatcher struct {
-	Epsilon float64
-	Value   string
-}
-
-func (NumericEpsMatcher) matcherNode() {}
-
-type AnyOrderMatcher struct {
-	Lines []string
-}
-
-func (AnyOrderMatcher) matcherNode() {}
-
-type NoMatcher struct{}
-
-func (NoMatcher) matcherNode() {}