Fixes #1055.

A parameter whose schema type union mixes a structured type (array/object) with a scalar has no well-defined style serialization: for type: [array, integer] a server cannot tell whether ?token=5 is ["5"] or 5. Valid JSON Schema, under-specified for OpenAPI serialization — so validate now reports it at WARN as ambiguous-parameter-serialization, an oasdiff-native lint alongside duplicate-enum-value.

Per the issue's scope:

• null is excluded from the union check — it carries no serialization, so [array, null] and [string, null] are unambiguous.
• Two scalars are fine ([string, integer]) — both serialize as a single value.

Implementation:

• The traversal is kin's openapi3.WalkParameters (openapi3: add T.WalkParameters to visit every parameter in a document getkin/kin-openapi#1224, written for this): every parameter in the document exactly once — components, path items, operations, callbacks, webhooks — with $ref dedup at the definition and RFC 6901 pointers as the finding Section. Mirrors how lintDuplicateEnums uses WalkSchemas.
• Source location points at the schema's type field via origin tracking; the id joins the validate rule registry (validate: fixed registry of rule IDs; gate derived IDs through it #1101).
• Content-based parameters (no schema) are skipped — no style serialization.
• Dependency note: bumps kin to a master pseudo-version pending the next kin release (brings the StringMap removal, hence the one-line mappingToStringMap type change — backward-compatible).

The issue's application/x-www-form-urlencoded request-body extension is left as the follow-up it suggests.