Include debug_metadata/*.natvis in the published crate to fix debugger_visualizer/docs.rs build

[teddytennant]

What

Add debug_metadata/*.natvis to the include allow-list in Cargo.toml so the natvis debugger-visualizer file is shipped in the published crate.

-include = ["Cargo.toml", "LICENSE-MIT", "LICENSE-APACHE", "README.md", "src/**/*.rs"]
+include = ["Cargo.toml", "LICENSE-MIT", "LICENSE-APACHE", "README.md", "src/**/*.rs", "debug_metadata/*.natvis"]

Why

Fixes #415.

The include allow-list added in #397 (to keep dev scripts out of the package) omitted debug_metadata/smallvec.natvis. But src/lib.rs references that file:

#![cfg_attr(
    feature = "debugger_visualizer",
    feature(debugger_visualizer),
    debugger_visualizer(natvis_file = "../debug_metadata/smallvec.natvis")
)]

Because the file is no longer packaged, the published 1.15.2 crate fails to compile whenever the debugger_visualizer feature is enabled:

error: couldn't read src/../debug_metadata/smallvec.natvis: No such file or directory (os error 2)

This is why the docs.rs build for 1.15.2 is red (docs.rs builds with --all-features on nightly), and it also breaks any downstream crate that enables the feature. Shipping the file fixes both, rather than just dropping the feature from docs.rs metadata. The tight debug_metadata/*.natvis glob keeps unrelated dev files out, consistent with #397's intent.

Testing

Metadata/packaging fix, so verified via cargo package:

• cargo package --list now lists debug_metadata/smallvec.natvis (previously absent).
• Building the packaged crate reproduces the bug before and confirms the fix after:
  • Before: cd target/package/smallvec-1.15.2 && cargo +nightly build --features debugger_visualizer fails with the couldn't read ...smallvec.natvis error above.
  • After: the same build succeeds.
• Existing suite unaffected: cargo test passes (no source change); cargo +nightly test --test debugger_visualizer --features debugger_visualizer compiles against the natvis file and passes.

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