feat(rollback): record per-component rollback state and namespace the rollback ref

[joshua-temple]

Problem

Rollback records state and eligibility against the flat, global env ladder. For a multi-component repo each component must roll back on its own state, its own environment subset, and its own ref namespace, without touching a sibling.

Fix

• Rollback for a selected component records state.components.<name>.<env> via WriteScopedState, preserving a sibling's state under the CommitWithRetry 409 re-apply.
• The rollback ref becomes rollback/<component>/<env> (rollback/<env> byte-identical when no component).
• First-environment eligibility and the previous-version ring are judged against the component's own environment subset, read from its overlaid state rows.
• An empty component keeps every path flat and byte-identical.

Scope is the rollback runtime. The generator emitting --component on the rollback workflow and the lifecycle e2e are separate changes.

Verification

go build ./..., go test ./... (2634 pass), go test -race on rollback+config (590 pass), golangci-lint run ./... clean; single-component byte-identical baseline gate green. Tests prove per-component state recording, a concurrent rollback-finalize 409 re-apply that preserves the sibling verbatim (including an unmodeled key), the namespaced ref, first-env scoped to the component's ladder, and the single-component path unchanged.

Refs #294.