Added interactive quantum circuit playground with tutorials

[CodeMaverick2]

Summary

Interactive digital quantum circuit builder that lets users visually build circuits, simulate quantum states, and execute on hardware. Complements the analog program builder (#5) to complete the teaching demo for Issue #3.

What's New

3 new modules:

• digital_simulator.py - quantum state simulation (kronecker product math, measurement sampling)
• digital_compiler.py - compiles Circuit → laser intensity patterns + trap commands
• gui/circuit_builder.py - interactive NiceGUI UI with circuit grid, gate palette, and results panel

Modified:

• digital.py - fixed pydantic v2 compatibility (root_validator → model_validator), relaxed qubit count to match 4-laser hardware
• gui/programs.py - added preset circuits (Bell state, GHZ, superposition, X gate)
• gui/main.py - wired "Quantum Playground" button to circuit builder

Features:

• 4×6 clickable circuit grid (qubits × time steps) with I/X/Z/H/CNOT gates
• Real-time quantum simulation with state vector display and probability bar charts
• Measurement sampling (100/1000 shots) with histogram visualization
• Hardware execution — lasers fire per gate, trap shakes during CNOT
• 4 guided tutorials: Superposition, Bit Flip, Entanglement (Bell State), GHZ State
• Export: "Show Circuit JSON" (OQD digital IR) and "Show Laser Program" (compiled hardware commands)
• No new dependencies (uses NiceGUI's built-in ECharts)

Demo -

Screen.Recording.2026-02-24.110521.1.mp4

Test Plan

• python src/oqd_teaching_demo/gui/main.py starts without errors (MOCK=True)
• All 4 tutorials load correct presets and step text
• Free Build: place/clear/reset gates, add/remove columns
• Simulate shows correct state vectors and probabilities
• Sample 100/1000 shots produces histogram
• Run on Hardware fires mock lasers + trap shake for CNOT
• Show Circuit JSON and Show Laser Program display correct output
• No new dependencies added

Closes - #3