Rename to packr, clean up flake/README/packages

[colinrozzi]

Summary

• Rename all crates from pack-* to packr-* (crate names, lib names, all Rust imports)
• Rewrite README.md — update from "Composite" to "Pack", modern code examples with packr_guest
• Update flake.nix — renamed package, added nix run .#test and nix run .#pr
• Clean up packages/ — fix sexpr dep (pack-abi → packr-abi), remove stale Cargo.lock files, add workspace exclude
• All workspace tests + derive tests pass, clippy clean
• 6/7 example packages build (typed-actor has pre-existing world!() macro issue)

Test plan

• cargo test --workspace passes
• cargo test -p packr-abi --features derive passes
• cargo clippy --workspace clean
• Example packages build (cargo check --target wasm32-unknown-unknown)

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