feat: implement capsule.compose@1 residual and validator

[Koh0920]

Summary

• define the canonical JCS CompositeResidual v1 selected by capsule.compose@1, including stable NodeId/Endpoint/Connection types, a 1 MiB bound, BLAKE3 ContentRef derivation, and golden vectors
• validate residual identity, exact parent exports, endpoint existence, protocol equality, protocol-owned role compatibility, transitive child resolution, and recursive compose closure
• make capsule.compose@1 explicitly linear and single-binding: one child endpoint may be used by at most one connection or export; fan-out requires an explicit Hub/Router Computation
• validate untrusted transitive closure with iterative DFS, configurable depth/unique-computation/resolved-byte budgets, and verified Computation/residual caches
• distinguish validation resource limit exceeded from semantic invalidity and stop before a CAS read that would exceed the aggregate byte budget
• allow cycles in the communication graph while rejecting recursive Computation reference cycles
• prove Core closure with a structural Hello World E2E: NameProvider and Greeter are internally connected while only greeting is exposed by one unchanged ComputationObject
• align the dependency-direction lint with the now-authoritative capsule-core DAG root introduced by refactor(ato): separate computation core from protocol v1 #1262

Scope boundaries

This PR implements canonical residual structure and structural conformance only. BoundaryVisibility::Internal means a connection is hidden from the parent boundary; it does not claim that complementary child actions synchronized or that a tau transition occurred.

This PR does not add or change PortGrant, live PortRef, Evaluator, small-step execution, Trace, Contract, Snapshot, materialization, or the Core ComputationObject shape. Runtime evolution, tau, and resealing remain follow-up work.

Verification

Passed locally at HEAD:

• cargo test -p capsule-compose --locked (14 tests: 3 codec + 11 structural/integration)
• 512-level compose closure validation through iterative DFS
• depth, unique-computation, and aggregate resolved-byte budget rejection tests
• 100 NodeIds sharing one ComputationRef perform one metadata read and one object open
• cargo check --workspace --locked
• cargo fmt --all -- --check
• cargo clippy -p capsule-compose --all-targets --locked -- -D warnings
• cargo clippy --workspace --all-targets --locked -- -D warnings
• bash scripts/check-dep-direction.sh

The earlier full cargo test --workspace --no-fail-fast run passed compose, Protocol v1 compatibility, Workspace PTY, Ready-State, and all other completed targets, but exited non-zero on the existing share_interactive_e2e and share_run_e2e fixtures because their committed share.spec.json / share.lock.json pair fails the pre-existing digest check. A serial rerun reproduced that unrelated fixture failure.

cargo clippy --workspace --all-targets --all-features remains blocked by the existing nacelle capsule_sync unresolved import under all-features; default-feature workspace Clippy passes.