promote each component through the environments independently

[joshua-temple]

Problem / why

The runtime promoter walks one global Config.Environments and writes one global
latest_release (internal/promote/promote.go, internal/promote/finalize.go). For a
multi-component manifest, the env walk and the finalize must run per component and write into
that component's state subtree. For a single-component manifest, the single default component
keeps today's behavior.

Scope (firm intent, details to be refined after V3)

• Make the promotion env walk a per-component loop; each component promotes through the
  environment ladder on its own version line.
• Port the finalize overlay to component-major: into.State.Components[comp][env] = mine...Components[comp][env], a disjoint top-level assign with no merge logic, and write a
  per-component latest_release.components[comp].
• CommitWithRetry stays unchanged; only the Mutate closure keys change. The serializer picks
  the single-component collapse or multi-component expand form per F3.

Files / areas touched

• internal/promote/promote.go, internal/promote/finalize.go.

Tests and coverage required

• Unit tests: promoting A does not alter B's state subtree or latest_release.
• e2e: promote one component dev to prod while the other stays put.
• A single-component regression test that promotion is unchanged.

Acceptance criteria

• Each component promotes on its own version line and writes only its own state subtree.
• Single-component promotion is unchanged.

Dependencies

Blocked by the independent versioning milestone (V1-V4).

[joshua-temple]

Implemented and merged via #517, #519, and #520.