feat(protocol): support directory project targets

[e54-bot]

Summary

• extend LPP project-loading to version 1.2 directory targets via entry.kind
• keep LPP 1.1 file-entry requests wire-compatible
• add normative directory-target semantics and conformance coverage

Validation

• cargo fmt --all -- --check
• cargo test --workspace
• ./target/release/lpp-conformance-runner --provider ./target/release/lpp-mock-provider --fixtures conformance/fixtures/v1

This is the protocol owner-side dependency for wrightkit/wright#317.

[Teakowa]

Two findings:

1. conformance/mock-provider/src/main.rs (documents_for_request / load_project): entry.kind is honored after negotiating either 1.1 or 1.2. A 1.1 session can therefore send kind: "directory" and receive the new 1.2 directory-target behavior, even though the spec defines directory targets as a 1.2 extension. This makes the version boundary non-enforceable and the claimed 1.1 compatibility unverified. Gate directory semantics on negotiated 1.2 and add conformance coverage showing that a 1.1 directory target is rejected rather than executed.

2. conformance/mock-provider/src/main.rs (load_project, directory branch): after selected.is_file(), selected.canonicalize().expect("entry exists") can still fail on a filesystem race or access error and panic the provider process. This path should return the structured projectLoadFailed error required by the protocol instead of terminating the process.

[Teakowa]

LGTM