docs: api_reference.md run_corners() return type CornerSweepResult is undocumented

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Problem

docs/api_reference.md documents run_corners() (around line 148) but only says:

Returns: CornerSweepResult containing results for all corners.

CornerSweepResult is never described anywhere in the file — no field descriptions, no type signatures, no usage example.

What the code shows

vajax/analysis/corners.py defines:

class CornerSweepResult:
    corners: list  # list of corner dicts
    results: list  # list of TransientResult

Users have no way to access the results without reading source code. At minimum the fields and their types should be documented, similar to how TransientResult is documented.

Note: Issue #109 covers the missing run_dc_sweep() and DCSweepResult — this is a separate gap for CornerSweepResult.