FXC-4546: add baseband source time classes for transient simulations

[dmarek-flex]

Note

Medium Risk
Introduces new source_time variants and updates schema discriminators and type unions used across simulation/source validation, which could affect serialization and downstream integrations if any assumptions about allowed SourceTimeType values exist.

Overview
Adds new baseband (carrier-free) source time types for transient RF simulations: BasebandStep, BasebandGaussianPulse, BasebandRectangularPulse, and BasebandCustomSourceTime (dataset-driven with FFT-derived bandwidth and end-time detection).

Wires these types through the system by introducing a shared SourceTimeType union in components/types/time.py, updating imports/validators and S-matrix component modelers/ports, exporting the new classes via tidy3d.__init__ and tidy3d/rf.py, and extending JSON schemas (Simulation, ModeSimulation, TerminalComponentModeler) so source_time discriminators accept the new variants.

Adds a comprehensive microwave test suite covering phase-locking to 0, waveform shapes, frequency_range/end_time behavior (including edge cases), plotting, and full Simulation validation with each new baseband type; also updates the CHANGELOG accordingly.

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[github-actions]

Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/init.py (100%)
• tidy3d/components/microwave/time.py (100%)
• tidy3d/components/source/base.py (100%)
• tidy3d/components/source/time.py (88.9%): Missing lines 110,115
• tidy3d/components/types/time.py (100%)
• tidy3d/plugins/smatrix/component_modelers/base.py (100%)
• tidy3d/rf.py (100%)

Summary

• Total: 149 lines
• Missing: 2 lines
• Coverage: 98%

tidy3d/components/source/time.py

Lines 106-119

  106         freqs = np.fft.rfftfreq(len(values), d=dt)
  107 
  108         peak = np.max(spectrum)
  109         if peak == 0:
! 110             return (0.0, 1.0)
  111 
  112         cutoff = np.exp(-(num_fwidth**2) / 2)
  113         above_cutoff = spectrum >= cutoff * peak
  114         if not np.any(above_cutoff):
! 115             return (0.0, freqs[-1])
  116 
  117         indices = np.where(above_cutoff)[0]
  118         fmin = float(freqs[indices[0]])
  119         fmax = float(freqs[indices[-1]])

---

[dmarek-flex]

Here are some nice pictures of the new baseband signals

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Schema type names mismatch Python class names

High Severity

The JSON schemas define type discriminators BasebandStepSourceTime and BasebandGaussianSourceTime, but the corresponding Python classes are named BasebandStep and BasebandGaussianPulse. This mismatch will cause JSON serialization/deserialization failures since the type discriminator values won't match between the Python code output and what the schemas expect.

Additional Locations (1)

• schemas/Simulation.json#L1433-L1470