Add slice_to_unit() C API for single-element slice detection

[Copilot]

This PR adds a CPython-exposed utility that collapses unit slices (e.g., slice(3, 4), slice(0, 1)) to their integer index and returns -1 for all non-unit slices. The goal is to provide a fast, explicit primitive for callers that can substitute scalar indexing when a slice semantically selects exactly one positive-position element.

• API surface

  • Added slice_to_unit() to the extension module method table.
  • Exported it through arraykit.__init__ and added typing in __init__.pyi:
    • def slice_to_unit(__slice: slice, /) -> int

• Core C implementation

  • Implemented slice_to_unit(PyObject *a) in src/methods.c (declared in src/methods.h).
  • Behavior:
    • Accepts only slice objects (TypeError otherwise).
    • Requires stop to be present and integral.
    • Accepts start is None and treats it as 0.
    • Accepts step is None or step == 1; any other step returns -1.
    • Returns start only when start >= 0, stop >= 0, and stop - start == 1; otherwise returns -1.

• Targeted coverage

  • Added focused tests in test/test_util.py for:
    • Positive matches: slice(3, 4), slice(0, 1), slice(None, 1)
    • Non-unit cases: width != 1, non-default step, missing stop, negative start
    • Type validation (TypeError on non-slice input)

from arraykit import slice_to_unit

slice_to_unit(slice(3, 4))      # 3
slice_to_unit(slice(0, 1))      # 0
slice_to_unit(slice(None, 1))   # 0
slice_to_unit(slice(0, 2))      # -1
slice_to_unit(slice(-1, 0))     # -1

[flexatone]

@copilot instead of returning None for non unit slices, lets return -1