test(rust): lock in path-qualifier preservation (#2)

[iray-tno]

Closes #2.

Background

The old "strip module qualifiers" step rewrote every X:: it saw, keeping only std/core/alloc. Because it inspected just the single token immediately before ::, it mangled valid paths:

• std sub-modules: std::collections::HashMap → std::HashMap, std::cmp::Reverse → std::Reverse
• associated functions: HashMap::new() → new(), Vec::new() → new()
• crate-internal paths: crate::foo::Item → crate::Item

…producing code that no longer compiled (E0432, E0425).

Resolution

As the issue itself recommended, the safest fix is to not strip qualifiers at all. Wrapping modules in mod name { ... } (#1, merged) makes every original path valid, so the stripping step was removed entirely there.

This PR locks that behavior in so it can't silently regress:

• Adds a qualified_paths fixture exercising exactly the issue's cases — std::collections::HashMap, std::cmp::Reverse, HashMap::new(), Vec::new(), and a crate-internal helpers::scale::by_two multi-segment path.
• Adds a regression test asserting all path segments survive preprocessing (and that the mangled forms like std::HashMap do not appear).
• Adds an rustc compile check on the output (skips automatically when rustc is unavailable).

Tests

Full suite green; the new fixture preprocesses with every qualifier intact and compiles/runs under rustc (edition 2021).

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