Support executable MastForest generation with syscalls, externals, and dyn nodes

### Feature description

The MastForest Arbitrary implementation generates non-executable forests when including certain node types:

1. **Syscalls**: Generated with random procedure digests - fail to execute without matching kernel procedures
2. **External nodes**: Use random digests that don't correspond to valid procedures
3. **Dynamic nodes**: Leave junk on the stack and cannot execute properly

Currently users must set these to 0 for executable forests, limiting execution testing using this strategy.

See https://github.com/0xMiden/miden-vm/pull/2335#discussion_r2486834656
https://github.com/0xMiden/miden-vm/pull/2335#discussion_r2486842056
https://github.com/0xMiden/miden-vm/pull/2335#discussion_r2486848817

### 1. Add kernel_procedures field to MastForestParams

```rust
pub struct MastForestParams {
// ... existing fields
/// Optional procedure hashes for syscall generation
pub kernel_procedures: Option>,
}
```

When generating syscalls, randomly select from provided hashes. Could also generate a matching kernel containing the procedure hashes.

### 2. Support executable external nodes

Add `external_procedures: Option>` field or make external nodes reference existing basic blocks within the same forest.

### 3. Consider kernel generation support

Add `generate_kernel: bool` field to generate both forest AND compatible kernel, returning `(MastForest, Kernel)`.