Add Features/ClassLabel schema views

[CarloLucibello]

What

Gives a dataset's schema a Julian home. ds.features now returns a Features view — an AbstractDict{String, Any} — instead of a raw Py mapping. Indexing a column yields a wrapped leaf:

julia> ds = ds.class_encode_column("label");

julia> cl = ds.features["label"]
ClassLabel(names=['neg', 'pos'])

julia> cl.names            # attribute/method access forwards to Python
2-element Vector{String}:
 "neg"
 "pos"

julia> cl.int2str(1)       # 0-based class id -> name
"pos"

ClassLabel/Value leaves are Py-backed and forward attribute/method access to Python (cl.names, cl.num_classes, cl.int2str, cl.str2int, v.dtype). Other feature types (nested Features, Sequence, Image, …) stay raw Py.

Highlights

• Access-site handling. A :features branch in Dataset's getproperty — the generic py2jl batch hot path is untouched (Features subclasses dict, so detecting it there would tax every batch observation).
• Tab-completion safe. Features caches its column names at construction, so keys/length/iteration never call Python (mirrors the DatasetDict fix).
• Round-trips into Python. Build a schema from Julia (ClassLabel(names=[…]), Value("int64"), Features(Dict(…))) and hand it back via jl2py as a features= argument.
• Label decoding in one call (public, unexported): class_names(ds, col), int2str(ds, col, i), str2int(ds, col, s). Class ids are 0-based data and pass through with no offset; the 0→1 bridge only appears when indexing the 1-based Julia names vector (names[ids .+ 1]), as documented.

API surface

Features and ClassLabel are exported; Value, features, class_names, int2str, str2int are public but unexported. The Pythonic idioms (ds.features, method chaining, datasets.ClassLabel(…) via the exported datasets handle) are the primary interface.

Tests & docs

• test/features.jl — 47 assertions, all local (no network), wired into runtests.jl. Full suite passes.
• API reference entries, a "Schema: features and labels" guide section, CHANGELOG + AGENTS.md notes. All doctests and the full docs build pass.

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