Feature/music play demo

[sedawwk]

Adds music_play_demo.c: it sends a text query that triggers the server's
music skill, parses the returned audio metadata, prints it, and downloads
the trial clip. Registers the tai_music_play_demo target in the POSIX
CMakeLists, and adds the user guide at docs-site/docs/tutorials/
music-play.md with the copyright notes on trial-clip limits and NetEase
Cloud Music integration.

Three things worth copying if you build on this demo, since example code
tends to be lifted into firmware verbatim:

• The trial clip is fetched with fork + execvp, never through a shell.
  The URL comes from the server, so passing it as one argv element is
  what makes $(...) in a URL inert; only http(s) schemes are accepted.

• Credentials from argv are length-checked before they reach
  iot_client_config_t's char[32] fields, which take no length argument
  and would otherwise be overflowed by a long value.

• "code" is read inside the SKILL envelope's data object. Resolved
  against the whole document, the common
  {"code":0,...,"data":{"code":"music"}} shape matches the outer status
  code first, and the music response is dropped without a word.
  try_parse_music() also distinguishes "not a music response" from "a
  music response whose payload could not be read", so the exit status
  can report the difference.

The metadata box pads by display column rather than by byte: the song
titles this demo exists to show are Chinese, and 开不了口 is 12 bytes but
8 columns, so "%-31s" would wreck the border.

Builds clean under -Wall -Wextra -Wshadow, and verified end to end
against the live service: the query returns a SKILL response, the
metadata parses, and the clip downloads as a valid MP3.