fix: normalize vendor-specific JSON keys to canonical schema names

[tiamilani]

Resolves #5 — some 3drp files use 'Theta_Tilt' instead of 'Theta_Electrical_Tilt', causing report generation to fail.

Checklist

• Merged latest main into this branch
• Lint and tests pass locally
• New/changed code has tests

What does this PR do?
Some antenna pattern files use a non-standard key name for the electrical tilt
(Theta_Tilt) instead of the schema-canonical Theta_Electrical_Tilt, which
caused report generation to fail. This normalizes known key variants when a
file is loaded, so these files parse and report correctly.

The canonical key is only set when it isn't already present, so files that
already use the standard name are unaffected and no value is overwritten.

Related issues
Closes #5

Notes for reviewers

• public API is unchanged.
• This branch contains the fix only. Its automated tests live on the stacked
  v0.2 test branch