#2418

Another follow-up to #2419. This introduces a Param struct to replace (String, Type) in Function.params, adding a Span field that tracks where each parameter name appears in the source WIT file.

Changes

• wit-parser: Add a public Param struct with name, ty, and span fields. The span is captured from the parameter name's Id::span during AST resolution. The span field is skipped during serialization, preserving the existing JSON format.
• wit-component: Add a ParamSignatures newtype in encoding/types.rs that wraps &[Param] with custom Hash/PartialEq implementations comparing only name and ty, ignoring span. This is needed because FunctionKey uses these traits to deduplicate component function types. Without it, two functions with identical signatures but different source locations would produce separate type entries in the encoded component. An alternative would be to have FunctionKey use &[Param] directly and then add the custom impls directly to it.
• wasm-wave, wit-dylib, wit-encoder: Update param construction and iteration to use the new Param struct.

Tests

• param_spans_point_to_names: Verifies that resolved param spans point exactly to the parameter names in source text.
• param_spans_preserved_through_merge: Verifies that param spans remain valid (is_known()) after merging two Resolve instances.