feat: add TOML codec and envc format support

New toml package (pelletier/go-toml/v2) matching yaml/json options and merge flow.
envc convert/get/merge accept --from/--to toml; fixture and CLI tests; docs refresh.

Co-authored-by: Cursor <cursoragent@cursor.com>

toml/codec.go

@@ -0,0 +1,117 @@
+package toml
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/eslider/go-config/internal/bytesutil"
+	"github.com/eslider/go-config/internal/keymap"
+	"github.com/eslider/go-config/internal/merge"
+	"github.com/eslider/go-config/internal/source"
+	"github.com/eslider/go-config/internal/structconv"
+	toml2 "github.com/pelletier/go-toml/v2"
+)
+
+// Codec loads TOML from multiple sources.
+type Codec struct {
+	sources    []source.Source
+	normalizer keymap.Normalizer
+	sliceStrat merge.SliceStrategy
+	structOpts structconv.Options
+	mergeOpts  []merge.Option
+	httpClient *http.Client
+	urlHeader  http.Header
+}
+
+// New creates a TOML codec.
+func New(opts ...Option) *Codec {
+	c := &Codec{
+		normalizer: keymap.LowerAlnum,
+		sliceStrat: merge.Replace,
+		structOpts: structconv.Options{
+			TagName:     "mapstructure",
+			WeaklyTyped: true,
+			Trim:        true,
+		},
+	}
+	for _, o := range opts {
+		o(c)
+	}
+	c.mergeOpts = []merge.Option{merge.WithSliceStrategy(c.sliceStrat)}
+	return c
+}
+
+// Map merges all TOML sources into one map.
+func (c *Codec) Map(ctx context.Context) (map[string]any, error) {
+	if len(c.sources) == 0 {
+		return nil, fmt.Errorf("toml: no sources")
+	}
+	acc := make(map[string]any)
+	for _, s := range c.sources {
+		b, err := bytesutil.ReadAll(ctx, s)
+		if err != nil {
+			return nil, fmt.Errorf("toml: read %s: %w", s.String(), err)
+		}
+		var parsed map[string]any
+		if err := toml2.Unmarshal(b, &parsed); err != nil {
+			return nil, fmt.Errorf("toml: parse %s: %w", s.String(), err)
+		}
+		if parsed == nil {
+			parsed = map[string]any{}
+		}
+		merge.DeepMerge(acc, parsed, c.mergeOpts...)
+	}
+	if c.normalizer != nil {
+		keymap.Walk(acc, c.normalizer)
+	}
+	return acc, nil
+}
+
+// Unmarshal decodes using context.Background.
+func (c *Codec) Unmarshal(dst any) error {
+	return c.UnmarshalContext(context.Background(), dst)
+}
+
+// UnmarshalContext decodes merged TOML into dst.
+func (c *Codec) UnmarshalContext(ctx context.Context, dst any) error {
+	m, err := c.Map(ctx)
+	if err != nil {
+		return err
+	}
+	return structconv.Decode(m, dst, c.structOpts)
+}
+
+// Marshal encodes src to TOML bytes (struct or map[string]any).
+func (c *Codec) Marshal(src any) ([]byte, error) {
+	m, err := encodeToMap(src)
+	if err != nil {
+		return nil, fmt.Errorf("toml: encode: %w", err)
+	}
+	var buf bytes.Buffer
+	enc := toml2.NewEncoder(&buf)
+	enc.SetIndentTables(true)
+	if err := enc.Encode(m); err != nil {
+		return nil, fmt.Errorf("toml: marshal: %w", err)
+	}
+	return buf.Bytes(), nil
+}
+
+// WriteTo writes TOML to w.
+func (c *Codec) WriteTo(w io.Writer, src any) (int64, error) {
+	b, err := c.Marshal(src)
+	if err != nil {
+		return 0, err
+	}
+	n, err := w.Write(b)
+	return int64(n), err
+}
+
+func encodeToMap(src any) (map[string]any, error) {
+	if m, ok := src.(map[string]any); ok {
+		return m, nil
+	}
+	return structconv.Encode(src)
+}