Accept Symbol property names in Fbe.overwrite

[Titanism0v0]

Closes #1243.

The single-property form of Fbe.overwrite rejected a Symbol property name before reading or writing the fact, while its Hash form accepts Symbol keys. This change converts a Symbol name to a String before the existing String check. String inputs retain their behavior, and unsupported names such as Integer and nil retain their existing errors.

Validation:

• Ruby 4.0.5, Bundler 2.5.16; locked dependencies satisfied.
• RED: the new missing-property and existing-property Symbol tests each raised the original Fbe::Error on the unmodified code.
• GREEN: bundle _2.5.16_ exec ruby -Itest test/fbe/test_overwrite.rb -- --no-cov passed (37 tests, 92 assertions); the non-String/non-Symbol compatibility test passed (1 test, 4 assertions).
• bundle _2.5.16_ exec rubocop lib/fbe/overwrite.rb test/fbe/test_overwrite.rb passed (2 files, no offenses); bundle _2.5.16_ exec rake yard passed.
• The full bundle _2.5.16_ exec rake remains red on both zero-patch and changed snapshots. Both reproduce the existing fake PR judges-action#193: add compare to Fbe::FakeOctokit #95 fixture mismatch, the root-only unwritable-directory test, and the award fixture returning 5 instead of 4. The changed snapshot reports 603 tests, 1637 assertions, 2 failures, 1 error, and 9 skips. The recorded baseline run reports 600 tests, 1627 assertions, 3 failures, 1 error, and 9 skips; its third failure was a one-off TestConsider#test_processes_fact_when_slot_fits_the_timeout timing failure.
• The isolated baseline rerun and the changed full run passed that timing test. The full rake is not reported as green.
• The full test suite passed after excluding only the three deterministic baseline failures (600 tests, 1631 assertions, 0 failures/errors, 9 skips). rake picks and full RuboCop also retain their separately confirmed baseline failures.