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/client/tracks.go

@@ -1,3 +1,4 @@
+// Package client provides methods for interacting with Yandex Music API endpoints.
 package client
 
 import (
@@ -8,7 +9,8 @@ import (
 	"gitea.mrixs.me/Mrixs/yandex-music-api-go/internal/utils"
 )
 
-// GetTracks gets tracks by IDs
+// GetTracks gets tracks by their IDs (GET /tracks).
+// Returns a slice of Track pointers, or nil if no IDs are provided.
 func (c *Client) GetTracks(ctx context.Context, trackIDs []string) ([]*models.Track, error) {
 	if len(trackIDs) == 0 {
 		return nil, nil
@@ -31,7 +33,8 @@ func (c *Client) GetTracks(ctx context.Context, trackIDs []string) ([]*models.Tr
 	return tracks, nil
 }
 
-// GetTrack gets a single track by ID
+// GetTrack gets a single track by ID using GetTracks.
+// Returns an error if no track is found with the given ID.
 func (c *Client) GetTrack(ctx context.Context, trackID string) (*models.Track, error) {
 	tracks, err := c.GetTracks(ctx, []string{trackID})
 	if err != nil {
@@ -43,7 +46,8 @@ func (c *Client) GetTrack(ctx context.Context, trackID string) (*models.Track, e
 	return tracks[0], nil
 }
 
-// GetTrackDownloadInfo gets download info for a track
+// GetTrackDownloadInfo gets download info for a track (GET /tracks/{track_id}/download-info).
+// The quality parameter specifies the desired audio quality (e.g., "mp3").
 func (c *Client) GetTrackDownloadInfo(ctx context.Context, trackID, quality string) ([]models.DownloadInfo, error) {
 	// This requires signing - use the sign utility
 	// For now, return basic implementation
@@ -58,7 +62,8 @@ func (c *Client) GetTrackDownloadInfo(ctx context.Context, trackID, quality stri
 	return result, nil
 }
 
-// DownloadTrack downloads a track
+// DownloadTrack downloads a track by ID.
+// It first fetches download info, finds a matching format/bitrate, then downloads the file content.
 func (c *Client) DownloadTrack(ctx context.Context, trackID, format string, bitrate int) ([]byte, error) {
 	info, err := c.GetTrackDownloadInfo(ctx, trackID, format)
 	if err != nil {
@@ -85,12 +90,12 @@ func (c *Client) DownloadTrack(ctx context.Context, trackID, format string, bitr
 	return c.request.Download(ctx, url)
 }
 
-// SearchResult represents search results
+// SearchResult represents search results containing tracks.
 type SearchResult struct {
 	Tracks *SearchTracksResult `json:"tracks"`
 }
 
-// SearchTracksResult represents track search results
+// SearchTracksResult represents track search results with total count and results list.
 type SearchTracksResult struct {
 	Total   int            `json:"total"`
 	Results []models.Track `json:"results"`