v0.1.9

Add Torque.get_many_defaults/3 for 0.1.9

Variant of get_many_nil/2 that takes %{path => default} and returns
%{path => value_or_default}. Saves callers from the awkward two-call
pattern:

  paths = Map.keys(defaults)
  values = Torque.get_many_nil(doc, paths)
  Enum.zip(paths, values)
  |> Map.new(fn {p, nil} -> {p, Map.get(defaults, p)}; pv -> pv end)

Now a single call:

  Torque.get_many_defaults(doc, %{
    "/user/id" => 0,
    "/user/name" => "anonymous",
    "/created_at" => nil
  })

Returns a same-shape map with parsed values, falling back to the
provided defaults for missing/null fields. Same nil/missing
indistinguishability semantics as get_many_nil/2.

Pure-Elixir wrapper around the existing NIF (get_many_nil/2); no
new Rust code, no NIF dispatch overhead beyond the one
get_many_nil call. Has an embedded doctest exercised by the test
suite.

D1's two other plan items resolved on inspection:

- 'aarch64 baseline variant in release matrix' -- aarch64 v8 is
  universal across consumer CPUs, no v2 distinction needed. The
  matrix already ships aarch64-apple-darwin + aarch64-linux-gnu.

- 'property test for float round-trip precision' -- already
  covered by test/property_test.exs (35 properties, 626 lines)
  with a json_scalar generator that includes float(-1000..1000),
  encoded/decoded through the round-trip suite.

README and mix.exs version both bumped to 0.1.9.