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/build.go

@@ -1,5 +1,10 @@
 package dsl
 
+import (
+	"maps"
+	"slices"
+)
+
 type BuildProfile int
 
 const (
@@ -105,9 +110,7 @@ func (dst *BuildConfig) MergeFrom(src *BuildConfig) {
 		if dst.Defines == nil {
 			dst.Defines = map[string]string{}
 		}
-		for k, v := range src.Defines {
-			dst.Defines[k] = v
-		}
+		maps.Copy(dst.Defines, src.Defines)
 	}
 }
 
@@ -132,24 +135,15 @@ func (b *BuildConfig) Resolve(defaults *BuildConfig, os string) BuildConfig {
 }
 
 func (b *BuildConfig) AppliesTo(os, compiler string) bool {
-	if len(b.Platforms) > 0 && !contains(b.Platforms, os) {
+	if len(b.Platforms) > 0 && !slices.Contains(b.Platforms, os) {
 		return false
 	}
-	if len(b.Compilers) > 0 && !contains(b.Compilers, compiler) {
+	if len(b.Compilers) > 0 && !slices.Contains(b.Compilers, compiler) {
 		return false
 	}
 	return true
 }
 
-func contains(xs []string, x string) bool {
-	for _, v := range xs {
-		if v == x {
-			return true
-		}
-	}
-	return false
-}
-
 type ToolchainSpec struct {
 	Name      string
 	Platforms []string