Plan AC and noise updates for MNA engine

[pepijndevos]

This implements AC analysis by:

• Unifying b vector to fully deferred pattern (like G/C matrices)
• Adding b_ac vector for AC excitation stamps
• Updating VoltageSource/CurrentSource with optional ac parameter
• Updating SPICE codegen to parse AC magnitude/phase specifications
• Creating new src/mna/ac.jl with ac!() and freqresp() functions

Key changes:

• MNAContext: Replace immediate b stamps with deferred b_I/b_V arrays
• MNAContext: Add b_ac_I/b_ac_V for AC disturbance tracking
• VoltageSource/CurrentSource: Add ac::ComplexF64 field
• stamp!(): Call stamp_b_ac!() for sources with AC specification
• ac!(): Build DSS system from linearized G, C matrices and b_ac
• freqresp(): Compute frequency response at any node

The AC analysis workflow:

1. Solve DC operating point
2. Linearize circuit at DC solution (G, C matrices)
3. Extract AC excitation vector (b_ac)
4. Build DescriptorStateSpace: E=C, A=-G, B=b_ac
5. Compute H(jω) = (jωC + G)⁻¹ · b_ac

Tests updated for new deferred b pattern and new AC API.