test: expand DSP, GUI, and notebook coverage

[Boussetta]

Closes #6

Summary

Expand automated coverage for the signal-processing core, edge cases, GUI behavior, and notebook structure.

Changes

• Cover every registered signal model, including AM, FM, and Chirp.
• Verify AM/FM bandwidth predictions and Fourier harmonic bandwidth expansion.
• Test exact Nyquist boundaries and below-Nyquist behavior.
• Test FFT reconstruction with both even and odd input lengths.
• Test short-duration sampling and minimum sample handling.
• Test anti-alias filter fallback when SciPy is unavailable.
• Test zero-error reconstruction metrics.
• Validate notebook JSON structure and local/Colab setup references.
• Add a GUI regression check for summary and context-aware control states.

Validation

• Full test suite: 38 passed
• Total coverage: 77%
• DSP coverage: 90%
• Signal coverage: 96%
• Ruff: passed
• mypy: no code errors
• git diff --check: passed

Commits

This PR intentionally contains focused commits:

• d087373 test: cover registered signal models and bandwidth
• c444799 test: cover DSP boundary and fallback behavior
• a915b53 test: validate notebook structure and setup

The generated .coverage file remains untracked and is not included.