feat: add the experimental-API mark (functions, methods, properties)

[TaiSakuma]

Implements step 1 of the implementation order proposed in discussion #4197 (design comment): the experimental mark for functions, methods, and properties — one shared wrapper.

One new public name: awkward.errors.ExperimentalWarning, a UserWarning subclass, because users reference it in warning filters. The decorator itself is internal (awkward._experimental.experimental).

Behavior, as specified in the design comment:

• @experimental and @experimental() are equivalent; the decorator takes no arguments in the initial release (any argument raises TypeError), and the parenthesized form is where configuration options can be added later.
• The mark sits directly above the def, below @property/@classmethod/@staticmethod, so it always receives a plain function.
• Everything user-visible derives from the decorated object: the name from __qualname__, the defining module from __module__.
• Each marked function warns once per process, on its first call — for a property, on its first access. The state is the wrapper's own, independent of the warnings filter state and registry.
• The warning's stacklevel is computed by walking the stack to the first frame outside awkward (the pattern of pandas' find_stack_level), so the reported location stays on the user's line even through @high_level_function() dispatch; a fixed number would attribute it to _dispatch.py. The warning also fires when a third-party array intercepts the operation through __awkward_function__, because the dispatcher calls the wrapper eagerly.

Example warning:

/home/user/analysis.py:12: ExperimentalWarning: stupendous_reduce is experimental.
    It may change or be removed in any release, without a deprecation period.
    Defined in: awkward.operations.ak_stupendous_reduce
  result = ak.stupendous_reduce(events)

One interaction worth recording: the module-level warnings.filterwarnings("default", module="awkward.*") in awkward._errors matches the frame a warning is attributed to, and this warning is attributed to user code, so that filter never governs ExperimentalWarning — visibility comes from UserWarning's default handling, and the once-per-process behavior from the wrapper's own state.

No real API is marked in this PR; it is machinery and tests only. 19 tests were written first and pin the message text, the warn-once behavior (including under simplefilter("always") and when the body raises), metadata preservation via functools.wraps (which high_level_function depends on for the dispatch name), stacklevel attribution directly and through generator-style and plain-function dispatch, and the warning under interception.

No docs change: sphinx-autoapi generates the class page automatically, matching the existing awkward.errors.FieldNotFoundError precedent.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (1b161d3) to head (ffcb6b4).

Additional details and impacted files

Files with missing lines	Coverage Δ
src/awkward/_experimental.py	100.00% <100.00%> (ø)
src/awkward/errors.py	100.00% <100.00%> (ø)

[github-actions]

The documentation preview is ready to be viewed at http://preview.awkward-array.org.s3-website.us-east-1.amazonaws.com/PR4279