refactor: standardize Go layout with internal terminalstate

Audit findings: cmd/webterm and webterm package were acceptable, but terminalstate
was an internal implementation detail exposed as a public package.

Changes:
- Move go/terminalstate -> go/internal/terminalstate
- Update imports to github.com/rcarmo/webterm-go-port/internal/terminalstate
- Keep package name terminalstate unchanged

Resulting package layout:
- github.com/rcarmo/webterm-go-port/cmd/webterm
- github.com/rcarmo/webterm-go-port/internal/terminalstate
- github.com/rcarmo/webterm-go-port/webterm

Validation:
- go test ./...
- go test -race ./...
- go coverage check remains 80.9%
- make check

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

go/webterm/coverage_boost_test.go

@@ -20,7 +20,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/rcarmo/webterm-go-port/terminalstate"
+	"github.com/rcarmo/webterm-go-port/internal/terminalstate"
 )
 
 func newUnixHTTPTestServer(t *testing.T, handler http.Handler) (string, func()) {

go/webterm/docker_exec_session.go

@@ -13,7 +13,7 @@ import (
 	"strings"
 	"sync"
 
-	"github.com/rcarmo/webterm-go-port/terminalstate"
+	"github.com/rcarmo/webterm-go-port/internal/terminalstate"
 )
 
 type DockerExecSpec struct {

go/webterm/session.go

@@ -1,7 +1,7 @@
 package webterm
 
 import (
-	"github.com/rcarmo/webterm-go-port/terminalstate"
+	"github.com/rcarmo/webterm-go-port/internal/terminalstate"
 )
 
 type SessionConnector interface {

go/webterm/svg_exporter.go

@@ -5,7 +5,7 @@ import (
 	"html"
 	"strings"
 
-	"github.com/rcarmo/webterm-go-port/terminalstate"
+	"github.com/rcarmo/webterm-go-port/internal/terminalstate"
 )
 
 var ansiColors = map[string]string{

go/webterm/svg_exporter_test.go

@@ -4,7 +4,7 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/rcarmo/webterm-go-port/terminalstate"
+	"github.com/rcarmo/webterm-go-port/internal/terminalstate"
 )
 
 func TestRenderTerminalSVG(t *testing.T) {

go/webterm/terminal_session.go

@@ -10,7 +10,7 @@ import (
 
 	"github.com/creack/pty"
 	"github.com/google/shlex"
-	"github.com/rcarmo/webterm-go-port/terminalstate"
+	"github.com/rcarmo/webterm-go-port/internal/terminalstate"
 )
 
 type TerminalSession struct {

go/webterm/test_helpers_test.go

@@ -3,7 +3,7 @@ package webterm
 import (
 	"sync"
 
-	"github.com/rcarmo/webterm-go-port/terminalstate"
+	"github.com/rcarmo/webterm-go-port/internal/terminalstate"
 )
 
 type fakeSession struct {