fix #44 fixed-array borrow lowering across modules

[Overcomplicated12]

Summary

Fix cross-module lowering for borrows of fixed-array locals.

When passing &local or &mut local where local has type [T; N], codegen now emits the bare local lvalue instead of applying slice/reference coercions. This preserves compatibility with emitted std::array<T, N>& and const std::array<T, N>& parameters.

Changes

• Detect stable fixed-array local borrows in common call-argument lowering.
• Emit dump32(v, b) and load32(b) for the cross-module varint case.
• Preserve existing lowering for rvalues, literals, fields, raw pointers, and non-array borrows.
• Add a crate-mode regression fixture that emits separate varint.cppm and serde.cppm modules.

Verification

• cargo test -p rusty-cpp-transpiler --test e2e_basic
• cargo test -p rusty-cpp-transpiler --bin rusty-cpp-transpiler

Fixes #44.