chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/alecthomas/chroma/v2/emitters.go

@@ -10,6 +10,12 @@ type Emitter interface {
 	Emit(groups []string, state *LexerState) Iterator
 }
 
+// ValidatingEmitter is an Emitter that can validate against a compiled rule.
+type ValidatingEmitter interface {
+	Emitter
+	ValidateEmitter(rule *CompiledRule) error
+}
+
 // SerialisableEmitter is an Emitter that can be serialised and deserialised to/from JSON.
 type SerialisableEmitter interface {
 	Emitter
@@ -30,6 +36,8 @@ type byGroupsEmitter struct {
 	Emitters
 }
 
+var _ ValidatingEmitter = (*byGroupsEmitter)(nil)
+
 // ByGroups emits a token for each matching group in the rule's regex.
 func ByGroups(emitters ...Emitter) Emitter {
 	return &byGroupsEmitter{Emitters: emitters}
@@ -37,6 +45,13 @@ func ByGroups(emitters ...Emitter) Emitter {
 
 func (b *byGroupsEmitter) EmitterKind() string { return "bygroups" }
 
+func (b *byGroupsEmitter) ValidateEmitter(rule *CompiledRule) error {
+	if len(rule.Regexp.GetGroupNumbers())-1 != len(b.Emitters) {
+		return fmt.Errorf("number of groups %d does not match number of emitters %d", len(rule.Regexp.GetGroupNumbers())-1, len(b.Emitters))
+	}
+	return nil
+}
+
 func (b *byGroupsEmitter) Emit(groups []string, state *LexerState) Iterator {
 	iterators := make([]Iterator, 0, len(groups)-1)
 	if len(b.Emitters) != len(groups)-1 {