docs: api_reference.md missing run_dc_sweep() and DCSweepResult

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Problem

api_reference.md has no documentation for run_dc_sweep() or DCSweepResult, even though the method is fully implemented and exported.

Confirmed the method exists:

from vajax import CircuitEngine
from vajax.analysis.engine import DCSweepResult
# Signature: (self, source: str, start: float, stop: float, step=None, points=None) -> DCSweepResult

DCSweepResult is also exported from vajax.analysis (in analysis/__init__.py), but not from the top-level vajax package.

Related

• Issue docs: getting_started.md and for_spectre_users.md incorrectly claim DC sweep is unavailable #104 covers getting_started.md and for_spectre_users.md incorrectly marking DC sweep as "Coming soon"
• This issue is specifically about the API reference being completely silent on the topic — no signature, no parameters, no example, no result type docs

Fix Needed

Add a run_dc_sweep() section to docs/api_reference.md documenting:

• Method signature and all parameters (source, start, stop, step, points)
• DCSweepResult dataclass fields (sweep_parameter, sweep_values, voltages, currents, num_points)
• A brief usage example