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/internal/terminalstate/tracker.go

@@ -0,0 +1,153 @@
+package terminalstate
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"sync"
+
+	"github.com/rcarmo/go-te/pkg/te"
+)
+
+var ansi16Names = [...]string{
+	"black",
+	"red",
+	"green",
+	"yellow",
+	"blue",
+	"magenta",
+	"cyan",
+	"white",
+	"brightblack",
+	"brightred",
+	"brightgreen",
+	"brightyellow",
+	"brightblue",
+	"brightmagenta",
+	"brightcyan",
+	"brightwhite",
+}
+
+type Cell struct {
+	Data       string `json:"data"`
+	FG         string `json:"fg"`
+	BG         string `json:"bg"`
+	Bold       bool   `json:"bold"`
+	Italics    bool   `json:"italics"`
+	Underscore bool   `json:"underscore"`
+	Reverse    bool   `json:"reverse"`
+}
+
+type Snapshot struct {
+	Width      int      `json:"width"`
+	Height     int      `json:"height"`
+	Buffer     [][]Cell `json:"buffer"`
+	HasChanges bool     `json:"has_changes"`
+}
+
+type Tracker struct {
+	mu                  sync.Mutex
+	screen              *te.DiffScreen
+	stream              *te.ByteStream
+	changeCounter       uint64
+	lastSnapshotCounter uint64
+}
+
+func NewTracker(width, height int) *Tracker {
+	screen := te.NewDiffScreen(width, height)
+	return &Tracker{
+		screen: screen,
+		stream: te.NewByteStream(screen, false),
+	}
+}
+
+func (t *Tracker) Feed(data []byte) error {
+	if len(data) == 0 {
+		return nil
+	}
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if err := t.stream.Feed(data); err != nil {
+		return err
+	}
+	if len(t.screen.Dirty) > 0 {
+		t.changeCounter++
+		// Clear dirty set so subsequent feeds detect new changes
+		for k := range t.screen.Dirty {
+			delete(t.screen.Dirty, k)
+		}
+	}
+	return nil
+}
+
+func (t *Tracker) Resize(width, height int) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if width == t.screen.Columns && height == t.screen.Lines {
+		return
+	}
+	t.screen.Resize(height, width)
+	t.changeCounter++
+}
+
+func (t *Tracker) Snapshot() Snapshot {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	snapshot := Snapshot{
+		Width:      t.screen.Columns,
+		Height:     t.screen.Lines,
+		HasChanges: t.changeCounter > t.lastSnapshotCounter,
+		Buffer:     make([][]Cell, t.screen.Lines),
+	}
+	t.lastSnapshotCounter = t.changeCounter
+
+	for row := 0; row < t.screen.Lines; row++ {
+		line := make([]Cell, t.screen.Columns)
+		for col := 0; col < t.screen.Columns; col++ {
+			raw := t.screen.Buffer[row][col]
+			data := raw.Data
+			if data == "" {
+				data = " "
+			}
+			line[col] = Cell{
+				Data:       data,
+				FG:         colorToString(raw.Attr.Fg),
+				BG:         colorToString(raw.Attr.Bg),
+				Bold:       raw.Attr.Bold,
+				Italics:    raw.Attr.Italics,
+				Underscore: raw.Attr.Underline,
+				Reverse:    raw.Attr.Reverse,
+			}
+		}
+		snapshot.Buffer[row] = line
+	}
+	return snapshot
+}
+
+func colorToString(color te.Color) string {
+	if color.Name != "" {
+		name := strings.ToLower(strings.TrimPrefix(color.Name, "#"))
+		if len(name) == 6 {
+			if _, err := strconv.ParseUint(name, 16, 32); err == nil {
+				return name
+			}
+		}
+		return name
+	}
+	switch color.Mode {
+	case te.ColorDefault:
+		return "default"
+	case te.ColorANSI16:
+		if int(color.Index) < len(ansi16Names) {
+			return ansi16Names[color.Index]
+		}
+		return "default"
+	case te.ColorANSI256:
+		return fmt.Sprintf("%d", color.Index)
+	case te.ColorTrueColor:
+		return "default"
+	default:
+		return "default"
+	}
+}