feat(mpd): add WriteXML and Period.Write/WriteToString

[tobbee]

Summary

• Extract the XML encoder logic from (*MPD).Write into an exported
  WriteXML(elem any, w io.Writer, indent string, withHeader bool) that can
  serialize any element. (*MPD).Write now delegates to it.
• Add (*Period).Write(w, indent) and (*Period).WriteToString(indent) for
  serializing a single Period fragment (no XML declaration).
• Clean up the doc comments on the new functions.
• Drop the goconst linter, which only flagged pre-existing repeated string
  literals (mostly test fixtures) across the package.

Tests

Added to mpd/io_test.go:

• TestWriteXML — WriteXML and (*MPD).Write produce identical output, the
  returned byte count matches bytes written, and withHeader toggles the
  <?xml declaration.
• TestPeriodWriteToString / TestPeriodWrite — a bare <Period> fragment
  renders with the expected attributes and no XML declaration, and Write
  agrees with WriteToString.

go test ./mpd/ and go vet ./mpd/ are green.

CHANGELOG

Added an Added entry under [Unreleased].

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