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fix(parser): unary minus, negative literals, and Error built-in type

- Accept leading `-` on int/float literals (including float tokens) and unary `-` in expressions; error on `-` before string, bool, nil, or array literals.
- Treat bare `Error` as the built-in error type so imports like `errors` align with signatures.

fix(typechecker): infer full tuple from `return f()` when arity matches

Splats callee return types when a single return expression is a call that returns multiple values.

fix(transformer): do not pad `return g()` with `, nil` for multi-value calls

Padding produced invalid Go (`return g(), nil`). Skip nil/zero padding when the sole expression is a call whose callee arity matches the function’s return arity.

feat(examples): tic-tac-toe uses ensure constraints and `(MoveResponse, Error)`

Example (constraints with negative literal):

````forst
ensure row is GreaterThan(-1) or invalidMove("row must be >= 0")
```

Example (multi-value return without bad padding):

```forst
func outer(): (Int, Error) {
	return inner()
}
```