Report missing statement semicolons at the closing token

Tracked by #346.

## Problem

A missing semicolon inside a function is currently collapsed into a generic top-level function error:

```wave
fun main() {
var value: i32 = 1
}
```

The parser has enough local context to report the missing `;`, but that detail is printed ad hoc and discarded.

## Scope

- Return a structured syntax diagnostic when a declaration or expression statement requires a semicolon.
- Point at the token that ends the incomplete statement, normally `}` or EOF.
- Include `;` in the expected-token list.
- Cover explicit and inferred `var` declarations.
- Add a valid statement control case.

## Completion criteria

- [ ] The message mentions the missing semicolon rather than the whole function declaration.
- [ ] The source location is the incomplete statement boundary.
- [ ] Human and JSON output agree.
- [ ] The parser writes no duplicate error line to stdout.

Start in the parser by reproducing the missing-semicolon example in the issue and trace how the local diagnostic is converted into the generic function error. Verify the declaration and expression-statement cases, explicit and inferred var forms, a valid control case, and both human and JSON output; done means the boundary token and expected `;` are reported once without stdout duplication.

Written by the indexing model from the issue text.