Flatten Rust binaries and fix README header title

[keys-i]

• Flatten Rust binaries from src/bin/<command>/main.rs to src/bin/<command>.rs.
• Fix the README header title.

Verification

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked — 50 passed
• git diff --check

Note

Binary flattening preserves Cargo’s existing command behavior.

[chloe41427]

Reviewed. This is a clean, low-risk mechanical change:

• Renames src/bin/<cmd>/main.rs → src/bin/<cmd>.rs for current, default, help, home, uninstall, version, with their snapshot tests moved alongside. All renames are 100% similarity — no logic changed, and Cargo's binary discovery treats both layouts identically, so command behavior is preserved.
• Fixes a duplicate H1 in the README (there were two # SDKMAN!... headers stacked).
• Net diff is trivial despite touching 21 files — almost entirely renames.
• CI green on macOS, Ubuntu, and Windows. cargo fmt, clippy -D warnings, and cargo test (50 passed) all verified per the PR description.

No concerns. Approving.

[marc0der]

Thanks for contributing @keys-i 🙏