[X03] Composition E2E

[yunseo-kim]

Scope

Add end-to-end composition fixtures for mutable, immutable same-run, foreign, partial, and linked-metadata paths.

Contracts

Composition specs and ADRs 0052, 0067, 0072, 0074

Dependencies

X02

Blocking issues: [X02] #37.

Executable QA

`go test ./internal/composition -run '^TestEndToEndCompositionScenarios$' -count=1 -v` must exit 0 with named subtests `mutable`, `immutable-same-run`, `foreign-conflict`, `partial-sidecar`, and `linked-metadata`. Call logs must prove no later mutation after each rejected tier.