Autobox Python tuple -> Julia Tuple

[jack-champagne]

Closes #12.

box_python_value handled bool/int/float/complex/str/None but not tuple, so a
Python tuple passed as an arg or kwarg value raised
TypeError: Cannot auto-convert tuple. This blocked tuple-valued Julia kwargs —
notably pypiccolo's SmoothPulseProblem(..., Δt_bounds=(0.1, 0.5)) and the
minimum_time demo.

Fix: recursively box each element (via auto_convert_arg, so nested tuples /
JuliaVal elements work) and build a Julia Tuple with new C jl2py_box_tuple
(Core.tuple(elts...), elements rooted across the constructing call). Output side
(Julia Tuple -> Python) already worked via the JuliaTuple wrapper.

Tests: tests/test_tuple_boxing.py — tuple identity, element access,
(0.1,0.5) -> Tuple{Float64,Float64}, tuple-as-kwarg-value, empty + nested.
Full suite: 85 passed, 3 skipped (Julia 1.12.6).

End-to-end validation against a real Piccolissimo bundle: with this change,
pypiccolo SmoothPulseProblem(qtraj, N, Q=100.0, R=1e-2, Δt_bounds=(0.1, 0.5))
now builds and evaluates (previously TypeError) — the exact pattern the
minimum_time demo needs.

🤖 Generated with Claude Code

[jack-champagne]

Pushed a follow-up commit addressing review feedback on the tuple-boxing code itself:

• box_python_value: match type(val) is tuple exactly instead of isinstance, so a collections.namedtuple (a tuple subclass) keeps raising TypeError instead of silently losing its field names as a flattened positional Tuple.
• jl2py_box_tuple: check/clear jl_exception_occurred() after jl_call, matching every other jl_call site in this codebase (previously the only one that didn't).
• jl2py_box_tuple: cache the Core.tuple function pointer in a static instead of re-resolving it on every call, matching getproperty_fn's existing pattern in the same file.

Added a namedtuple regression test; full suite is 86 passed / 3 skipped (up from 85/3).

Separately opened #14 for a GC-rooting issue in JuliaVal.__call__ and a few container call sites that predates this PR (multiple freshly-boxed values sitting unrooted in a Python list before a single consuming call) — out of scope for this PR since it's not tuple-specific, but worth a look independently.