feat(routing): calibrated hybrid semantic search scoring, explicit search modes, and test bench simulator overhaul

[spelech]

Summary

This PR aligns Model Context Gateway's semantic search and simulator with ContextCortex (PR #36), introducing calibrated hybrid scoring, explicit search modes, decomposed score diagnostics, and a redesigned Semantic Router Simulator.

1. Calibrated Hybrid Scoring & Explicit Search Modes (MCP-40, MCP-41, MCP-42)

• Scoring Formula: Normalized linear combination model:
  374453\text{Score} = \alpha \cdot \text{DenseScore} + (1.0 - \alpha) \cdot \text{SparseScore}374453
• Configurable Dense Weight: Dynamic dense weight $\alpha \in [0.0, 1.0]$ (default: 0.5 balanced).
• Search Modes:
  • hybrid: Balanced or weighted combination of dense vector similarity and normalized lexical matching.
  • semantic: Pure dense SIMD vector similarity ($\alpha = 1.0$).
  • lexical: Pure sparse / token keyword matching ($\alpha = 0.0$, bypassing embedding generation).
• Score Decomposition: ToolSearchResult exposes Score, DenseScore, SparseScore, DenseRank, and SparseRank.
• Meta-Mode search_tools: MCP tool declaration and execution updated with optional mode and dense_weight parameters with 100% backward compatibility for existing callers.
• REST API Endpoint: POST /api/test/semantic-search receives mode, denseWeight, and limit, returning an envelope with decomposed results.

2. Semantic Router Simulator Redesign (UI-142, UI-143)

• 3-Way Search Mode Toggle: Segmented buttons for Hybrid Fusion, Semantic (Vector), and Lexical (Keyword).
• Interactive Hybrid Split Slider & Presets: Conditionally rendered in hybrid mode with a live ratio indicator (Semantic X% / Keyword Y%) and 5 quick calibration presets:
  • Balanced (50/50)
  • Semantic Bias (70/30)
  • Keyword Bias (30/70)
  • Pure Semantic (100/0)
  • Pure Keyword (0/100)
• Result Limit Selector: Dropdown for 5, 10, 15, or 25 tools.
• Enhanced Hit Cards: Decomposed score badges (Score: XX.X%, Semantic: XX.X%, Keyword: XX.X%), server badges, and formatted markdown descriptions.
• 1-Click "Test Tool" Action: Instantly transitions to the Tools tab and preselects the server and tool in the Tool Tester form.

3. Documentation & Live Real Screenshots

• Captured live screenshot of the updated simulator: docs/assets/semantic_router_simulator.jpg.
• Updated docs/user-guide/test-bench/semantic-search.md and ARCHITECTURE.md with scoring diagrams, formula, modes, and API guides.

4. Verification & Quality Gates

• Backend .NET Tests: 909/909 tests passing (including unit & integration tests for calibrated hybrid search, modes, and endpoints).
• Frontend Vitest: 47/47 test files passing (300/300 tests passing).
• Frontend ESLint: 0 errors, 0 warnings.
• Frontend Build: SPA production bundle compiled cleanly in wwwroot/.
• Playwright E2E: Multi-viewport layout audits passing with 0 overflow across desktop and half-ultrawide viewports (1720x1440 and 1280x1080).
• SRS Requirements Catalog: Verified zero drift (464 requirements across 1078 proofs).
• Version Bump: Bumped to 5.21.0 across ModelContextGateway.csproj, frontend/package.json, useUserStore.ts, CHANGELOG.md, and README.md.