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/lexer.go

@@ -130,6 +130,23 @@ type Lexer interface {
 	AnalyseText(text string) float32
 }
 
+// Trace is the trace of a tokenisation process.
+type Trace struct {
+	Lexer   string  `json:"lexer"`
+	State   string  `json:"state"`
+	Rule    int     `json:"rule"`
+	Pattern string  `json:"pattern"`
+	Pos     int     `json:"pos"`
+	Length  int     `json:"length"`
+	Elapsed float64 `json:"elapsedMs"` // Elapsed time spent matching for this rule.
+}
+
+// TracingLexer is a Lexer that can trace its tokenisation process.
+type TracingLexer interface {
+	Lexer
+	SetTracing(enable bool)
+}
+
 // Lexers is a slice of lexers sortable by name.
 type Lexers []Lexer