sourcewalk introduces blocks

### MWE:
```
using MacroTools
using MacroTools: textwalk

code = """
Rule(x -> -sin(x))
Rule(x -> 1 + tan(x)^2)
""";

after = textwalk(code) do expr
@capture(expr, Rule(v_)) && return v
return expr
end

println(after)
```

### Output:
```
(x->begin
-(sin(x))
end)
(x->begin
1 + tan(x) ^ 2
end)
```

Start by running the MWE in the issue and tracing the textwalk/sourcewalk path in MacroTools. Compare the transformed expressions with the reported output; done means the walk no longer introduces the unexpected begin blocks while preserving the intended expression structure.

Written by the indexing model from the issue text.