fix tip ordering for untyped Altair shorthand axes (0.2.1)

[jbloom]

Charts whose strain axis used positional shorthand without a type suffix — alt.Y("strain") instead of alt.Y("strain:N") — silently rendered in data order instead of tree-tip order. The field-extraction helper called to_dict() on the bare channel, which raises when the shorthand has no explicit type because Altair needs a chart-data context to infer it. The blanket except Exception returned None, the live-object walker found no match, and the sort override was silently skipped.

Replace the to_dict-based reader with one that inspects _kwds directly, handles plain-str fields, untyped/typed/aggregate shorthand, and the FieldName(SchemaBase) wrapper that from_dict introduces; raise on unrecognized shapes instead of swallowing them.

Add a structural fail-fast tripwire: count strain-axis encodings on the spec walk and on the live-object walk, and raise on any mismatch. Catches both this bug and any future divergence between the two surfaces.

Consolidate the two near-identical live-object walkers (sort application and axis suppression) into a single read-only generator yielding live channel objects; the call site in _build applies sort and (conditionally) axis suppression at each yielded channel and counts hits for the tripwire. Removes ~70 lines of duplicated traversal logic.

Tests: 11 new unit and end-to-end tests covering typed/untyped/ explicit/aggregate shorthand, the from_dict-roundtripped wrapper, value-only encodings (legitimate None), unparseable shorthand (raises), non-channel objects (raises), tip ordering for untyped shorthand on x and y, and the tripwire firing when the live walker misses a strain encoding the spec walker found.