port updated harfbuzz classdef size calculation code

See https://github.com/harfbuzz/harfbuzz/pull/4616

The previous impl would produce incorrect results in certain instances, although it would take a very pathological input for this to actually result in a packing failure. Our impl is here: https://github.com/googlefonts/fontations/blob/main/write-fonts/src/graph/splitting/pairpos.rs#L385

Compare the updated HarfBuzz classdef size calculation in PR 4616 with the implementation at write-fonts/src/graph/splitting/pairpos.rs:385. Trace how the current implementation handles pathological inputs, then confirm the port matches the corrected behavior and no longer permits incorrect packing results.

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