[C5 HL-030] Prove and extract a reusable composition engine boundary #40

Outcome

Move reusable mechanics out only when a second real work or independent lifecycle proves the boundary.

Placement and activation

• Circle: C5 — Reusable and operated product platform
• System responsibilities: Product / Platform
• Capability coverage: 01 Core Domain / Proprietary Engine; 03 APIs, SDKs & Integrations; 09 Developer Platform & Tooling; 16 Meta / Portfolio Orchestration
• Planning status: Conditional; activate only when the requirement below is met.
• Activation: A second real consumer or independently governed lifecycle exists.
• Parent epic: [EPIC C5] Reusable and operated product platform #8; master roadmap: [MASTER EPIC] Hole-loop: concentric evolution within Persona / Visual Form Canon #2.

Dependencies

Required prerequisites:

• HL-004 #13: a documented boundary, component ownership, and composition contract.
• HL-005 #15: the source/score contract used by the proposed consumers.
• HL-010 #20: exact-version runtime and verification evidence relevant to the shared mechanism; public deployment is required only if the selected consumer needs a live service.
• HL-045 #14: inspected sibling ownership and compatible real-consumer evidence.

Conditional dependencies:

• HL-032 #42, HL-033 #43, HL-034 #44, and HL-035 #45 apply only if the extracted boundary actually operates hosted persistence, accounts, a service runtime, or availability obligations respectively. A local library can be extracted and contract-tested without introducing those capabilities.

The two-real-consumer requirement for a new shared typography/composition library remains mandatory. Independent lifecycle or permission evidence can justify another component boundary; it does not by itself prove shared-library reuse.

Acceptance evidence

• Inventory existing estate capabilities before building replacements; evaluate reuse against timing, graph, rendering, state/replay, and edition needs.
• Demonstrate a second corpus/score consumer using the same mechanism while keeping hole-loop meaning and source custody project-specific.
• Define package/service ownership, versioned interfaces, semver compatibility, contract tests, license, changelog, release gates, and migration/dependency policy.
• Record the lifecycle/deploy/permission/reuse evidence and actual consumer before core/engine extraction; establish how the product composes pinned versions.
• Keep the historic and standalone editions runnable through the migration; publish rollback and compatibility evidence.
• Demonstrate compatibility with an actual named Persona consumer through its invariant. Time-moment calibration remains private and does not become the source owner or universal media runtime; research repositories do not count as second runtime consumers.
• A new shared typography/composition library requires at least two actual consumers with proven contract compatibility; related research or a prepared repository target does not satisfy that evidence.

Completion and scope

Attach the actual artifact or commit, relevant verification, limitations, and an artist decision where a composition changes. This issue plans authorized work; creation does not claim implementation or deployment. Preserve independent work/source identities and private source boundaries. Repository creation requires the extraction evidence in HL-004 #13 and HL-045 #14.