feat(text2sql): add stream runtime and semantic readiness

[LienJack]

Summary

This PR brings the broader Text2SQL feature branch together for main: chat streaming now has a shared protocol toolkit, frontend cancellation is wired through the backend runtime path, semantic asset readiness becomes part of the Text2SQL/RAG flow, and SQLite validation has stronger dry-run diagnostics. It also refreshes the project README as the primary English entry point with Chinese and Japanese translations plus functional screenshots.

These ship together because the stream protocol, runtime evidence, semantic context, validation behavior, and README now describe the same end-to-end Text2SQL delivery surface.

flowchart TB
  UI["Frontend chat workbench"] --> Protocol["chat-stream-protocol package"]
  Protocol --> API["NestJS chat stream API"]
  API --> Runtime["Text2SQL v2 LangGraph runtime"]
  Runtime --> Knowledge["Knowledge-owned RAG and semantic assets"]
  Runtime --> Validation["SQL validation and dry-run execution"]
  Runtime --> Delivery["Delivery contract and evidence"]
  Delivery --> UI

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What Changed

Area	Result
Chat streaming	Adds a shared @text2sql/chat-stream-protocol package for SSE framing, parsing, terminal guards, and UI projection helpers.
Cancellation	Propagates AbortSignal from frontend stop generation through backend workflow, LangGraph, SQL generation, and LLM provider paths.
Semantic readiness	Adds manifest-first semantic asset preparation, reindex readiness, retrieval payload contracts, and RAG audit/quality coverage under the knowledge domain.
Validation	Improves SQLite dry-run validation and Text2SQL v2 validation diagnostics with new unit/integration coverage.
Delivery evidence	Extends delivery mapping, stream event projection, and focused eval gates so v2 artifacts and semantic context stay traceable.
Frontend chat	Refines assistant composer, thinking panel, message rendering, result tabs, API client behavior, and related test coverage.
Documentation	Rewrites README.md in English, adds README.zh-CN.md and README.ja.md, and includes feature screenshots for modeling, datasource onboarding, ChatBI, SQL evidence, and chart output.
Graph metadata	Refreshes graphify output and trims report whitespace so branch diff checks pass.

Screenshots

Verification

• git diff --check origin/main..HEAD

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