Add comprehensive godoc documentation to all exported symbols

- Document all exported types, functions, methods, and constants across 52 files
- Add package-level doc comments to client, models, utils, and ynison packages
- Document HTTP method and endpoint path for each API client method
- Document model struct fields and DeJSON/DeList/ToJSON/ToMap methods
- Fix duplicate package doc comment in internal/models/base.go

Co-Authored-By: Claude Code <noreply@anthropic.com>

internal/models/track.go

@@ -1,10 +1,11 @@
+// Package models provides data models for the Yandex Music API.
 package models
 
 import (
 	"encoding/json"
 )
 
-// Track represents a full track
+// Track represents a full track from the Yandex Music API.
 type Track struct {
 	BaseModel
 	ID                       string           `json:"id"`
@@ -27,7 +28,7 @@ type Track struct {
 	DownloadInfo             []DownloadInfo   `json:"download_info"`
 }
 
-// DeJSON deserializes a Track from JSON
+// DeJSON deserializes a Track from JSON data.
 func (t *Track) DeJSON(data []byte, c ClientInterface) (Model, error) {
 	var raw map[string]json.RawMessage
 	if err := json.Unmarshal(data, &raw); err != nil {
@@ -115,7 +116,7 @@ func (t *Track) DeJSON(data []byte, c ClientInterface) (Model, error) {
 	return track, nil
 }
 
-// DeList deserializes a list of Tracks
+// DeList deserializes a list of Tracks from JSON data.
 func (t *Track) DeList(data []byte, c ClientInterface) ([]Model, error) {
 	var rawList []json.RawMessage
 	if err := json.Unmarshal(data, &rawList); err != nil {
@@ -133,7 +134,7 @@ func (t *Track) DeList(data []byte, c ClientInterface) ([]Model, error) {
 	return result, nil
 }
 
-// GetDownloadInfo returns download info for a specific quality
+// GetDownloadInfo returns download info for a specific quality, or nil if not found.
 func (t *Track) GetDownloadInfo(quality string) *DownloadInfo {
 	for _, info := range t.DownloadInfo {
 		if info.Quality == quality {
@@ -146,14 +147,14 @@ func (t *Track) GetDownloadInfo(quality string) *DownloadInfo {
 // Note: Artist and Album types are defined in their respective files
 // These are imported automatically since they're in the same package
 
-// Major represents major info
+// Major represents major label info for a track.
 type Major struct {
 	ID          string `json:"id"`
 	Name        string `json:"name"`
 	Description string `json:"description"`
 }
 
-// TrackLyricsInfo represents minimal lyrics info for a track
+// TrackLyricsInfo represents minimal lyrics info for a track.
 type TrackLyricsInfo struct {
 	ID        string `json:"id"`
 	Text      string `json:"text"`
@@ -162,7 +163,7 @@ type TrackLyricsInfo struct {
 	HasText   bool   `json:"has_text"`
 }
 
-// DownloadInfo represents file download info
+// DownloadInfo represents file download info for a track.
 type DownloadInfo struct {
 	Quality    string `json:"quality"`
 	Codec      string `json:"codec"`