Subckt files for different ICs

[anikjain11]

Purpose

This PR adds 11 verified subcircuit models and KiCad schematic symbols for widely used analog and mixed-signal ICs to enhance eSim's component library.

Subcircuits Included:

• Data Converters: 3-bit Flash ADC
• Amplifiers & Buffers: LT1010 (Buffer), OPA656 (Wideband Op-Amp), LMH6702 (High-Speed Op-Amp)
• Fully Differential & Variable Gain Amps: THS4541, THS4303, VCA821
• Specialized RF/Analog: AD8307 (Logarithmic Amp), AD834 (4-Quadrant Multiplier), CPPLL (Charge Pump PLL)

Verification Approach

Each IC went through the following pipeline:

1. Schematic creation in KiCad and ERC validation.
2. Netlist generation and conversion to NgSpice subcircuits with appropriate parameter mappings.
3. Transient and AC analysis plotting within eSim to verify output accuracy against official datasheets.