Is there a better way to have a boxed part letter at the beginning of an arbitrary line?

[Kevin Cole]

A minimum working example will make the question clearer: Although the
following produces what I want, it seems very ugly, and I suspect
there's a better way to accomplish the same thing: Placing the [A] and
[B] boxed letters for the tune parts at the beginning of a line.

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\version "2.24.2"
\language "english"

melody = \relative c' {
  \time 6/8
  \key d \major
  \clef treble

  d8^\markup \translate #'(-10 . 0) { \box \pad-markup #0.5 "A" }[ e8
e8] g8[ a8 b8] |
  \break
  d8^\markup \translate #'(-10 . 0) { \box \pad-markup #0.5 "B" }[ e8
e8] g8[ a8 b8] |
  \bar "|."
}

\score {
  \new Voice = "mel" { \melody }

  \layout {
    \autoBeamOff
    indent = 0\mm
  }
}

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