Restructure into a crates/ workspace layout

[MattJackson]

What

Move all five Rust crates under a single crates/ directory, so code is cleanly separated from non-code at the repo root:

crates/{busbar, api, auth-tokens, auth-admin-tokens, hooks-ranking}
examples/  scripts/  docs/  bench/  config.yaml  providers.yaml  Dockerfile   ← non-Rust, at root

Previously the binary sat at the root (src/ + the root Cargo.toml package) with the satellite crates scattered as auth/tokens, hooks/ranking, etc. — which made it hard to tell code from non-code at a glance. This is the layout newcomers expect: crates/ = Rust, everything else = not.

Why

Pure navigability. The dependency architecture is unchanged (busbar → api ← plugins; each plugin is an optional, default-on feature). It just reads more clearly.

Notes

• No behavior change. The binary and every crate are functionally identical; only file paths move. Git tracked the moves as renames, so history is preserved.
• No version bump, no changelog entry, no tag — this is an internal reorganization, invisible to anyone running the binary. It rides along in the next real release.
• Root Cargo.toml is now a virtual workspace manifest; [profile.release] stays there (Cargo reads profiles only from the workspace root). Each crate's [package] is in crates/<name>/Cargo.toml.
• Four CARGO_MANIFEST_DIR test reads gained ../../ to reach the repo-root providers.yaml/config.yaml.
• scripts/structure-lint.sh now scans crates/; docs/code-layout.md documents the workspace layout.
• The website's sync-docs.mjs reads the crate version from crates/busbar/Cargo.toml (updated in the marketing repo, with a root fallback so it works across the transition).

Verification (local)

• cargo test — full suite passes
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• cargo build --no-default-features — clean
• scripts/structure-lint.sh — passes