feat(v0.3): frequency-domain tools — margin(), rlocus(), pole_placement()

[Oseiasdfarias]

Overview

Add the missing frequency-domain and pole-placement tools to complete the MATLAB-compatible API surface.

Functions

Function	Description
margin(sys)	Gain margin, phase margin, crossover frequencies
rlocus(sys)	Root locus plot as gain K varies
pole_placement(A, B, poles)	Ackermann / place() — state feedback gain for desired poles
pade(delay, n)	Padé approximation of pure time delay

API

from synapsys.api import margin, rlocus, pole_placement

gm, pm, wgc, wpc = margin(G)
K = pole_placement(A, B, desired_poles=[-2, -2+1j, -2-1j, -5])

Acceptance Criteria

• All functions pass mypy strict
• 100% test coverage
• Results match scipy.signal and MATLAB reference values (within 1e-6)