)

makeLeftRight typeset the inner math list twice — once to size the
delimiters and once to place the content. Typesetting an MTMathList is
not idempotent: preprocessMathList fuses adjacent ordinary atoms in
place via MTMathAtom.fuse (nucleus += atom.nucleus), so the second pass
re-fused already-fused runs and duplicated any multi-atom content inside
a \left…\right group. Nesting compounded the effect (one extra pass per
level).

Typeset the inner list once, cache the resulting MTMathListDisplay, and
reuse it for both the delimiter-height calculation and placement.

Adds LeftRightDuplicationTests covering the issue mgriebling#66 reporter
expression plus fraction, bracket, nested, and \left. variants.