Attribute identity is (namespace URI, local name): remove local-name conflation from lenient matching and candidate arrays #841

Principle

Attribute identity is (namespace URI, local name). Two attributes with different identity are different attributes — always. Two attributes with the same identity on one element are not well-formed XML (Unique Att Spec) and are rejected by the parser. There is no third case.

Corollary per Namespaces in XML: the default namespace never applies to attributes — an unprefixed attribute is always in no namespace, even when the document's xmlns is the namespace the producer "meant". So type can never be xmi:type; a producer intending the XMI-namespaced attribute must use a prefix.

Where we conflate today (to remove)

1. Lenient local-name fallbacks (find_attribute_by_local_name): match by local part across namespaces — distinct attributes collapse when a rule probes by bare name. Undeclared-prefix recovery (Lenient parsing of out-of-namespace documents silently drops elements (gml:description vanishes on round-trip) #754) is the one legitimate use; beyond that, a multi-match is an ambiguity, and should surface as such rather than silently resolve (today: order-of-attributes-dependent).
2. Candidate-array resolution in find_attribute_value(Array): rule spellings are resolved first-array-match; Fix attribute name in document order #837 proposes document-order-last. Both are resolution policies for what should be exact namespaced-name rules. Under exact identity, an element matching two candidates is a mapping ambiguity, not a choice to arbitrate by position or array order.

The motivating case (ea / XMI)

xmi:type (metaclass discriminator, XMI namespace) and type (classifier reference, no namespace) are different attributes. EA's ownedParameter profile writes only type; standard UML XMI writes xmi:type. Consumers should declare two exact rules and get both attributes distinctly — the ea gem currently carries a serialization-time regex (plain_parameter_type) solely because the model layer collapses them into one slot.

Proposal

1. Mapping rules are exact: map_attribute binds to (namespace URI, local name) — prefix-independent (the URI is the identity; the document's prefix spelling is not).
2. Lenient local-name matching is a recovery mechanism only: fires solely for the undeclared-prefix case; when a lenient probe matches MORE THAN ONE distinct attribute, return/signal ambiguity (log + no value, or a strict-mode error) instead of picking one.
3. find_attribute_value(Array) semantics: keep for alias lists where at most one alias can exist (spelling variants of one name); document that two simultaneous matches are an authoring error and surface them.
4. Consumers (ea/xmi gem) declare xmi:type and type as separate rules; the single-slot collapse and its regex workaround retire.

Non-goals

• Document-order or array-order arbitration of conflated matches (treats the symptom; makes collapse deterministic instead of absent).
• Supporting "attributes in the default namespace" — not expressible in XML.