feat(showcase): add replay-first trace evaluation fixtures

[christso]

Bead: av-vwa.1

Summary:

• Add replay-first trace evaluation showcase fixtures, targets, proof script, deterministic graders, and docs.
• Include imported Codex transcript fixture generated through existing import plumbing.
• Prove replay target fixture lookup runs without live LLM calls while graders run fresh.

Verification from worker handoff:

• bun examples/showcase/trace-evaluation/scripts/prove-replay.ts
• transcript import eval via bun apps/cli/src/cli.ts eval ...
• bun run validate:examples
• bun run test
• bun run typecheck
• focused Biome check for showcase files

Integration review: branch is 0 behind / 1 ahead of current origin/main.