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/compiler.go

@@ -3,6 +3,7 @@ package runner
 import (
 	"fmt"
 	"runtime"
+	"slices"
 	"sort"
 	"strings"
 
@@ -161,7 +162,7 @@ func compileMSVC(cfg dsl.BuildConfig, tc Toolchain, outputPath string) []string
 		argv = append(argv, "/W4", "/permissive-")
 	}
 
-	if containsString(cfg.Sanitize, "address") && cfg.Profile != dsl.ProfileSanitized {
+	if slices.Contains(cfg.Sanitize, "address") && cfg.Profile != dsl.ProfileSanitized {
 		argv = append(argv, "/fsanitize=address")
 	}
 
@@ -195,12 +196,3 @@ func sortedKeys(m map[string]string) []string {
 	sort.Strings(keys)
 	return keys
 }
-
-func containsString(xs []string, x string) bool {
-	for _, v := range xs {
-		if v == x {
-			return true
-		}
-	}
-	return false
-}