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/options.go

@@ -0,0 +1,87 @@
+package toml
+
+import (
+	"io"
+	"net/http"
+
+	"github.com/eslider/go-config/internal/keymap"
+	"github.com/eslider/go-config/internal/merge"
+	"github.com/eslider/go-config/internal/source"
+	"github.com/go-viper/mapstructure/v2"
+)
+
+// Option configures a Codec.
+type Option func(*Codec)
+
+// WithBytes appends TOML from bytes.
+func WithBytes(b []byte) Option {
+	return func(c *Codec) { c.sources = append(c.sources, source.Bytes{Data: b, Name: "bytes"}) }
+}
+
+// WithReader appends TOML from r.
+func WithReader(r io.Reader) Option {
+	return func(c *Codec) { c.sources = append(c.sources, source.Reader{R: r, Name: "reader"}) }
+}
+
+// WithFile appends a TOML file path.
+func WithFile(path string) Option {
+	return func(c *Codec) { c.sources = append(c.sources, source.File{Path: path}) }
+}
+
+// WithURL appends TOML from an HTTP(S) URL.
+func WithURL(raw string) Option {
+	return func(c *Codec) {
+		var hdr http.Header
+		if c.urlHeader != nil {
+			hdr = c.urlHeader.Clone()
+		}
+		c.sources = append(c.sources, source.URL{Raw: raw, Header: hdr, HTTPClient: c.httpClient})
+	}
+}
+
+// WithHTTPClient sets the client for subsequent WithURL sources.
+func WithHTTPClient(client *http.Client) Option {
+	return func(c *Codec) { c.httpClient = client }
+}
+
+// WithHTTPHeader adds a header for subsequent WithURL sources.
+func WithHTTPHeader(k, v string) Option {
+	return func(c *Codec) {
+		if c.urlHeader == nil {
+			c.urlHeader = make(http.Header)
+		}
+		c.urlHeader.Add(k, v)
+	}
+}
+
+// WithKeyNormalizer sets key normalizer after merge (nil disables).
+func WithKeyNormalizer(n keymap.Normalizer) Option {
+	return func(c *Codec) { c.normalizer = n }
+}
+
+// WithSliceMerge sets slice merge strategy across sources.
+func WithSliceMerge(s merge.SliceStrategy) Option {
+	return func(c *Codec) { c.sliceStrat = s }
+}
+
+// WithTrim toggles string trim on struct decode.
+func WithTrim(enable bool) Option {
+	return func(c *Codec) { c.structOpts.Trim = enable }
+}
+
+// WithWeaklyTyped toggles weak typing for struct decode.
+func WithWeaklyTyped(enable bool) Option {
+	return func(c *Codec) { c.structOpts.WeaklyTyped = enable }
+}
+
+// WithTagName sets mapstructure tag name.
+func WithTagName(name string) Option {
+	return func(c *Codec) { c.structOpts.TagName = name }
+}
+
+// WithDecodeHook appends a decode hook.
+func WithDecodeHook(h mapstructure.DecodeHookFunc) Option {
+	return func(c *Codec) {
+		c.structOpts.ExtraHooks = append(c.structOpts.ExtraHooks, h)
+	}
+}