Parse named proto contract implementations

Tracked by #348.

## Scope

Extend the parser and source AST with the named contract form:

```wave
proto Printable : Buffer {
fun print(self) {
}
}
```

Keep this distinct from the existing inherent extension form:

```wave
proto Buffer {
fun clear(self: ptr) {
}
}
```

## AST requirements

Preserve the proto name, target type, target generic arguments, method bodies, visibility/source provenance, and whether the block is inherent or named.

## Completion criteria

- [ ] Both proto forms parse without ambiguity.
- [ ] Missing proto name, colon, target, braces, or method body has a structured diagnostic.
- [ ] Existing colon-free proto tests remain unchanged.
- [ ] Parser-only positive and negative tests are added.
- [ ] Contract coherence, constraints, and method resolution remain outside this parser task.

Start by reading the parser and source AST handling for the existing colon-free proto form, then inspect its parser-only tests. Add coverage for named and inherent forms, including the listed malformed inputs; done means both forms preserve the required AST data and produce structured diagnostics without changing existing tests.

Written by the indexing model from the issue text.