v0.3.0: Restructure PID core, interactive drone demo, improved dashboard

[darioalessandro]

Summary

• PID core rewrite: Split monolithic lib.rs into modules (compute.rs, config.rs, controller.rs, state.rs, enums.rs, error.rs, thread_safe.rs). Pure pid_compute() function, validated ControllerConfig builder, derivative-on-measurement, IIR derivative filter, back-calculation anti-windup, no_std support.
• Interactive drone demo: Replaced hand-rolled ASCII charts with a ratatui TUI game. Real-time altitude control with missions, wind gusts (W/S), setpoint changes (Up/Down), and live PID gain tuning (1-6). PID term decomposition chart shows P/I/D contributions. Help screen (H) explains controls.
• Pidgeoneer dashboard: Dark theme, Chart.js with hover tooltips, PID term decomposition, intro section explaining the HVAC simulation.
• Demo script: Auto-starts Iggy via Docker (pinned to iggyrs/iggy:0.4.203), polls for readiness, proper cleanup.
• Cleanup: Removed redundant temperature_control and multithreaded_control examples, removed unused rand dependency.

Test plan

• cargo test — 14 unit tests + 8 doc tests pass
• cargo fmt --all -- --check
• cargo clippy -- -D warnings (including examples)
• cargo build --examples
• cargo check --no-default-features (no_std)