feat(validation): verify expected run observations

[morluto]

Summary

• adds expected observations to validation definitions and persisted run results
• evaluates those observations against base and candidate artifacts
• exposes observation results through CLI and MCP output without executing corpus reads

PR #68 merged into its stacked base after that base had already landed on main. This PR carries the resulting validation-only diff onto main.

Closes #48

Review order

1. Evidence contracts and observation evaluation
2. Corpus persistence and artifact safety
3. Application, CLI, and MCP integration

Verification

• go test ./... -count=1
• go test ./internal/evidence ./internal/corpus ./internal/app ./internal/cli ./internal/mcpserver -count=1

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