ADR-061: Schema entry points for concern-split output

[nathanacurtis]

Summary

• Implements ADR-061: adds JSON Schema entry points for --split-concerns CLI output, which previously had none.
• New types ComponentApi, ComponentVariants, ComponentExamples (concern-scoped subsets of Component), each with a per-component schema and a library-wide set-wrapper schema, mirroring the existing component.schema.json / components.schema.json pairing.
• root.schema.json's oneOf grows from 2 to 8 entries.
• Purely additive — lands within the already-reserved 0.28.0 (no version bump needed, package.json already at 0.28.0).

Test plan

• tsc -p tsconfig.build.json --noEmit — clean
• validate-schema.sh — 11/11 schemas pass
• Manual cross-file $ref resolution check — all pointers resolve
• tests/*.test-d.ts (including new ComponentConcerns.test-d.ts) — compiles clean, no regressions
• Author review of diff, then /specs.adr.accept to mark ADR-061 ACCEPTED