refactor: move embedded tests into dedicated test directory

docs/development.md

@@ -12,6 +12,7 @@ cargo test -p crossterm color_scheme_notifications
 cargo test -p crossterm --doc
 cargo clippy --all-targets -- -D warnings
 cargo fmt --package vamp --check
+rustfmt --edition 2024 --check tests/unit/battery_macos.rs tests/unit/clipboard_macos.rs
 for file in src/ruby_*.rb examples/config.rb examples/plugins/greeting.rb; do
   ruby -c "$file" || exit
 done
@@ -34,7 +35,9 @@ Rust embeds the Ruby implementation at compile time: [`ruby_values.rb`](../src/r
 
 ## Test organization
 
-Rust tests live alongside their modules; native LSP integration checks are in [`tests/lsp.rs`](../tests/lsp.rs).
+Rust tests live under [`tests/`](../tests/). Private unit tests are in [`tests/unit/`](../tests/unit/), linked from their owning modules with `#[cfg(test)]` and `#[path]`; nested macOS modules use `include!` to retain their private/platform scope. Root `tests/*.rs` files remain Cargo integration targets, including native LSP checks in [`tests/lsp.rs`](../tests/lsp.rs).
+
+`cargo test` runs both groups; `cargo test --lib` and `cargo test --bin vamp` select the library and CLI unit suites. Do not name a unit file `tests/unit/main.rs`: Cargo would discover it as a separate integration target.
 
 - Editor checks cover Unicode editing, multi-target state, undo/redo, saving failures, and modal paste acceptance and rejection atomicity.
 - Visual-command checks preserve directed characterwise/linewise ranges and multiple carets through command cancellation and plugin dispatch, exercise remapped command entry, and submit `:diff.comment` on either historical side. Live PTY smoke selects lines, cancels a prompt, then runs `:diff.comment` directly and verifies the saved original range without using the palette.