feat: implement capsule.compose@1 small-step reduction

[Koh0920]

Goal

Given semantically valid child transition evidence, one sealed and valid C_init forks by interaction into two independently sealed futures:

• C_init --name?"Alice"--> C_A1 --tau--> C_A2 --greeting!"Hello, Alice!"--> C_A3
• C_init --name?"Bob"--> C_B1 --tau--> C_B2 --greeting!"Hello, Bob!"--> C_B3

All seven Composite Computations are content-addressed and structurally valid with the same external { name, greeting } boundary, without changing Capsule Core. Alice is not a default: C_A1 is the residual computation produced by the earlier name?"Alice" interaction.

Changes

• Add pure generic small-step reducers for internal, connected, and exported child actions.
• Require ValidatedComposite as reducer input: valid state → transition → candidate residual → seal → revalidate.
• Reject same-node connections structurally; retain BoundaryVisibility::Internal separately from semantic Tau.
• Model parent name as an exported input through NameProvider.input.
• Fork Alice and Bob branches from the exact same C_init.
• Preserve each input through NameProvider.name ↔ Greeter.name synchronization and lift the corresponding greeting output.
• Assert distinct branch identities and exact ReadyName(...) / ReadyGreeting(...) residual bytes.
• Introduce no missing-input or default-name transition.

Validation

• cargo test -p capsule-compose
• cargo clippy -p capsule-compose --all-targets --all-features --locked -- -D warnings
• cargo check --workspace --locked
• cargo test --workspace --no-fail-fast (passed before this test-only canonical-scenario update)