refactor(conformance): introduce StoryboardStepHintBase for kind+message invariant

[bokelley]

Closes #950

Summary

• Adds StoryboardStepHintBase<K extends string> interface (no = string default) to src/lib/testing/storyboard/types.ts
• ContextValueRejectedHint extends StoryboardStepHintBase<'context_value_rejected'>, removing the duplicate kind and message declarations
• Exports StoryboardStepHintBase from storyboard/index.ts and src/lib/testing/index.ts alongside StoryboardStepHint and ContextValueRejectedHint
• Includes patch changeset

What was tested

• tsc --noEmit on the lib tsconfig shows only pre-existing environment errors (@types/node missing, deprecated moduleResolution); zero new type errors introduced
• ContextValueRejectedHint extends the base with a concrete literal — discriminated union narrowing confirmed unaffected
• buildHint object literal in rejection-hints.ts continues to typecheck (explicit kind: 'context_value_rejected' satisfies the base + concrete fields)

Notes

• Wire format unchanged — TypeScript-only structural change
• StoryboardStepHintBase<string> is the recommended supertype for fallback renderers (see JSDoc on the interface)
• The = string default was intentionally omitted: callers must supply the literal type argument, preventing future kinds from accidentally inheriting kind: string and losing discriminant narrowing

Pre-PR review:

• code-reviewer: approved — no breaking change, discriminated union narrowing intact, buildHint typechecks; nit on type keyword in inner barrel unfounded (already inside export type { block)
• dx-expert: approved — export placement correct, StoryboardStepHintBase<string> supertype pattern is sound; two nits (inline JSDoc example, comment explaining absent default) noted for follow-up

Session: https://claude.ai/code/session_01P4h7SnRH8N5djMswLnFzf7

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Generated by Claude Code

[bokelley]

Superseded by #959 — landed there as part of the structured StoryboardStepHint taxonomy.

[bokelley]

Superseded by #959 — StoryboardStepHintBase landed there as part of the structured StoryboardStepHint taxonomy.