Add VideoKnowledgeBase blackboard scaffolding and pipeline-side population

[Copilot]

VidPipe’s current stage-by-stage pipeline keeps analysis outputs siloed and forces repeated detection work. This PR introduces an additive blackboard foundation (VideoKnowledgeBase) so analysis artifacts can be shared across stages/agents without disrupting current execution flow.

• Blackboard contract (typed, additive)

  • Added VideoKnowledgeBase and related domain types in src/L0-pure/types/index.ts.
  • Extended PipelineResult with optional knowledgeBase to expose shared analysis state to downstream consumers.
  • Exported VideoKnowledgeBase from src/index.ts for SDK/public usage.

• KB persistence module

  • Added src/L6-pipeline/knowledgeBase.ts with:
    • createVideoKnowledgeBase(video)
    • writeVideoKnowledgeBase(videoDir, kb)
    • readVideoKnowledgeBase(videoDir)
  • Standardized persistence target as knowledge-base.json in each video directory.

• Pipeline integration (backward-compatible)

  • processVideo() now initializes KB post-ingestion and incrementally populates available fields (layout metadata, transcript, shorts, medium clips) alongside existing outputs.
  • KB is persisted at pipeline end with non-fatal error handling to avoid changing existing failure semantics.
  • processVideo() returns the populated knowledgeBase in PipelineResult.

• Focused test coverage

  • Added src/__tests__/unit/L6-pipeline/knowledgeBase.test.ts for KB init + read/write behavior.
  • Updated src/__tests__/unit/L6-pipeline/pipeline.test.ts to assert PipelineResult.knowledgeBase content.

// New shared contract exposed in pipeline output
const result = await processVideo('/videos/test.mp4')

result.knowledgeBase?.transcript?.merged   // normalized transcript source for downstream agents
result.knowledgeBase?.layout.webcam        // single detected webcam region
// persisted to: <videoDir>/knowledge-base.json