ICE in `FunctionDefinition::type` when an `external` free function is bound via `using for`

The compiler panics with `Solidity assertion failed` in [`libsolidity/ast/AST.cpp:490`](https://github.com/ethereum/solidity/blob/9be66192f9db40e5a43f71fc78e06b2f35e9ccdd/libsolidity/ast/AST.cpp#L490) inside `FunctionDefinition::type()` when a free function declared `external` is listed in a `using { f } for T` directive.

MRE:
```solidity
function f(int) external;
using {f} for int global;
```

Reproduce: save as `mre.sol`, run `solc --bin mre.sol`.

Output:
```
Internal compiler error:
/solidity/libsolidity/ast/AST.cpp(490): Throw in function virtual const solidity::frontend::Type* solidity::frontend::FunctionDefinition::type() const
Dynamic exception type: boost::wrapexcept
std::exception::what: Solidity assertion failed
[solidity::util::tag_comment*] = Solidity assertion failed
```

Expected behavior: a type error rejecting `external` free functions in `using for` bindings, since only `internal` and `public` free functions make sense as bound methods.

Git commit: 9be66192f
solc 0.8.35-develop.2026.4.18+commit.9be66192.Linux.g++

Start in libsolidity/ast/AST.cpp at FunctionDefinition::type(), then reproduce the ICE with `solc --bin mre.sol` using the issue's minimal Solidity example. Trace validation of `using {f} for int global` and ensure an `external` free function produces a type error rather than an assertion; add regression coverage for this case.

Written by the indexing model from the issue text.