Merge #43: Add API subcommand

d0abec0 cargo-rbmt: extend api command with baseline arg (Nick Johnson)
a105adc cargo-rbmt: add API command (Nick Johnson)
dfcf054 cargo-rbmt: refactor get_packages to return tuples (Nick Johnson)

Pull request description:

  This is a little more complex than the other `rbmt` commands. Up to now, `rbmt` has just orchestrated `cargo` commands, no other tool dependencies. But this patch adds an `api` command which takes over [the exposed API checks](https://github.com/rust-bitcoin/rust-bitcoin/blob/a4d9d21d4fa0c10099e356cd401eac668c042eca/contrib/check-for-api-changes.sh) and uses an internal dependency.

  The legacy scripts in rust-bitcoin use the `cargo-public-api` tool has a nice library crate, `public-api`. This patch uses the library crate under the hood for `rbmt`'s `api` command.

  The `api` command has two modes which I think models mostly the usage in rust-bitcoin. The standard `cargo rbmt api` generates exposed API files for all packages. It can use the global `-p` flag to filter which packages. It checks to make sure the feature flags are additive and if anything has changed. The second mode is `cargo rbmt api --baseline $REF` which takes a git ref and does a comparison. It fails if there are any breaking changes.

ACKs for top commit:
  tcharding:
    ACK d0abec0

Tree-SHA512: 41a40828b63d890e3aedb7decd4277670350048db7eec27ac6d2a4a3e877046eeaf4beb74d72f65fe14f14ee2c1ba78308efa101bd87964dcecd3693a089c905

Author: tcharding

Cargo.lock

@@ -16,3 +16,4 @@ clap = { version = "4", features = ["derive"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 toml = "0.8"
+public-api = "0.47"

cargo-rbmt/Cargo.toml

@@ -115,10 +115,6 @@ To ensure your package works with the full range of declared dependency versions
 
 The `lock` command generates and maintains these files for you. You can then use `--lock-file` with any command to test against either version set.
 
-### Usage
-
-**Generate/update lock files**
-
 ```bash
 cargo rbmt lock
 ```
@@ -127,8 +123,6 @@ cargo rbmt lock
 2. Generate `Cargo-minimal.lock` with minimal versions across the entire dependency tree.
 3. Update `Cargo-recent.lock` with conservatively updated dependencies.
 
-**Use a specific lock file**
-
 ```bash
 # Test with minimal versions.
 cargo rbmt --lock-file minimal test stable
@@ -143,6 +137,26 @@ cargo rbmt --lock-file minimal docs
 
 When you specify `--lock-file`, the tool copies that lock file to `Cargo.lock` before running the command. This allows you to test your code against different dependency version constraints.
 
+## API Checking
+
+The `api` command helps maintain API stability by generating public API snapshots and checking for breaking changes. It uses the [public-api](https://github.com/Enselic/cargo-public-api) crate to analyze a crate's public interface.
+
+```bash
+cargo rbmt api
+```
+
+1. Generates API snapshots for each feature configuration (no-features, alloc-only, all-features).
+2. Validates that features are additive (enabling features only adds to the API, never removes).
+3. Checks for uncommitted changes to API files.
+
+The generated API files are stored in `api/<package-name>/`.
+
+```bash
+cargo rbmt api --baseline v0.1.0
+```
+
+Compares the current API against a baseline git reference (tag, branch, or commit) to detect breaking changes.
+
 ## Workspace Integration
 
 `cargo-rbmt` can simply be installed globally on a system or added as a dev-dependency to a package.