roll a single component back independently

[joshua-temple]

Problem / why

Rollback uses a global Previous ring and a global rollback ref namespace
(internal/rollback/rollback.go). For a multi-component manifest, rolling back one component
must not affect siblings.

Scope (firm intent, to be refined after P1)

• Per-component Previous ring under state.components[comp][env].
• Segment the rollback ref namespace per component: RollbackRefPrefix becomes
  rollback/<comp>/<env>.
• Port the rollback Apply overlay to component-major.

Files / areas touched

• internal/rollback/rollback.go.

Tests and coverage required

• e2e: rolling back A leaves B's state and refs untouched.
• A single-component regression test that rollback is unchanged.

Acceptance criteria

• A per-component rollback touches only that component's ring, refs, and state subtree.
• Single-component rollback is unchanged.

Dependencies

Blocked by the independent promotion milestone. Can land alongside R1.

[joshua-temple]

Implemented and merged via #525.