perf: Remove unused length computation in List.myers_difference

[tomciopp]

myers_difference_with_diff_script/3 computed
length(list1) + length(list2) and threaded it through find_script/4,
which never read it: the recursion is bounded by each_diagonal/5
finding the endpoint, which the Myers algorithm guarantees within
n + m iterations. Two full list traversals of dead work per call,
also paid by String.myers_difference/2 and ExUnit's diff output.

Assisted by: Claude Fable.

[tomciopp]

Mix.install([{:benchee, "~> 1.4"}])

tag = System.get_env("TAG") || raise "set TAG, e.g. TAG=main or TAG=p03"
result_file = fn t -> Path.join(__DIR__, "p03_myers.#{t}.benchee") end
baselines = Path.wildcard(Path.join(__DIR__, "p03_myers.*.benchee")) -- [result_file.(tag)]

identical = Enum.to_list(1..1000)
small_edit = List.replace_at(identical, 500, :changed)
disjoint_a = Enum.to_list(1..200)
disjoint_b = Enum.to_list(1001..1200)

str_a = String.duplicate("abcdefghij", 100)
str_b = String.replace(str_a, "abcdefghij", "abcdeXghij", global: false)

Benchee.run(
  %{
    "identical 1000" => fn -> List.myers_difference(identical, identical) end,
    "one edit 1000" => fn -> List.myers_difference(identical, small_edit) end,
    "disjoint 200 (control)" => fn -> List.myers_difference(disjoint_a, disjoint_b) end,
    "String.myers_difference" => fn -> String.myers_difference(str_a, str_b) end
  },
  warmup: 1,
  time: 3,
  memory_time: 1,
  save: [path: result_file.(tag), tag: tag],
  load: baselines
)