chore: add crates.io publishing and release tooling - #30

summary

adds proper release tooling for crates.io and github releases:

• adds keywords and categories to all crate manifests for discoverability
• adds make publish target that publishes crates in dependency order
• adds make publish-dry-run to verify all crates can be packaged
• includes 30s delay between publishes for crates.io indexing

usage

# verify packaging works
make publish-dry-run

# publish to crates.io (requires `cargo login`)
make publish

# create github release for current version
make github-release

# bump version and tag
make release-patch  # 0.1.3 -> 0.1.4
make release-minor  # 0.1.3 -> 0.2.0

test plan

• cargo check passes
• cargo publish --dry-run -p ember-protocol works (crate already exists on crates.io)