docs: rewrite README.md with comprehensive project documentation

[Copilot]

README was a bare skeleton that didn't reflect the project's architecture, current state, or technical decisions.

Rewrote to include:

• Why Taran — positioning vs JMeter/K6/Goose, key differentiators (zero GC, Coordinated Omission handling, single binary)
• Architecture diagram — ASCII art showing the full component flow: CLI → Config → Execution Engine (VUs) → Protocol Clients → Metrics → Reporters
• Workspace structure — table mapping all 7 crates to their responsibilities with dependency flow description
• Scenario configuration reference — all 4 load profiles (constant, ramp, stepped, spike), step definitions, assertions, extractors
• Current status — honest split between implemented features and planned phases (1–6)
• Technology stack — linked table of all key dependencies (Tokio, reqwest, rustls, Rhai, hdrhistogram, etc.)
• Development guide — prerequisites, build/test/lint commands, project conventions (strict Clippy, no unwrap, conventional commits)

Content sourced from docs/PLAN.md, CLAUDE.md, and the actual crate implementations.

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