Root freshly-boxed call args immediately, not after the whole list

[jack-champagne]

Summary

box_python_value's box_* calls return a jl_value_t* with nothing on
the Julia side keeping it alive. JuliaVal.__call__ (positional args and
kwargs), JuliaArray.__setitem__/jl_getindex, and JuliaDict.__setitem__
all box multiple values via separate ctypes calls and only root them once
the full list is handed to the consuming Julia call — leaving each
already-boxed element GC-invisible while later elements are still being
boxed.

If a GC cycle runs in that window, an earlier-boxed value can be collected
and its memory reused by a later allocation, corrupting it silently.

auto_convert_and_root (new in convert.py) roots each freshly-boxed
value via the existing jl2py_ref_create/jl2py_ref_release machinery
(already used by JuliaValGC — no new C code) the instant it's boxed,
released once the call completes. Existing JuliaVal args are left as-is
since they're already kept alive by the caller's own Python reference.

Reproduced directly before writing the fix: boxing 5 floats one at a
time with a forced full GC (GC.gc(true)) between each box call reused
the same freed memory for the next box, corrupting every element after
the first. Small ints are unaffected — Julia's boxed-int cache means
jl_box_int64 returns an already-rooted, permanent pointer for small
values, so this only bites floats/strings/complex/large ints (e.g. any
call with 2+ float args, which is common in pypiccolo's numeric code).

Test plan

• Existing suite: 80 passed, 3 skipped (baseline on master, unchanged)
• With fix: 83 passed, 3 skipped (+3 new tests in tests/test_arg_rooting.py)
• New tests force a GC cycle between each argument's boxing (monkeypatching
  auto_convert_and_root) to make the race deterministic, covering the
  positional-args, kwargs, and dict-__setitem__ paths
• Confirmed the new tests fail (can't even import the API) against
  unfixed master, and confirmed via a standalone ctypes-level repro
  that unfixed code silently corrupts values under forced GC pressure