docs(crates): fix public repository links

[leonardosaracchi5]

What changed

• Replace the crates/README.md link to the private/missing plans/rust-takeover.md file with the public TypeScript interop boundary.
• Replace the link to the missing archived scafld spec with the same current public boundary document.
• Update the architecture link label so it matches the public repository layout.

Why

The Rust workspace guide retained paths from the project’s earlier monorepo layout. In the public checkout, both referenced files are absent, so readers hit dead links while trying to understand the native CLI cutover.

Impact

Contributors can now follow every local Markdown link in the Rust workspace guide to current, public documentation.

Validation

• git show --check
• Repository-wide scan of local Markdown link targets: all targets resolve

[auscaster]

Checked the diff against current main. Both replacement documents exist, the old rust-takeover and archived scafld links are stale public references, and the change keeps the contributor-facing crates guide on current canonical docs.