Skip virtual columns when generating cached fixture INSERTs (#55)

Models with generated/virtual columns reject INSERTs into those
columns ("cannot INSERT into generated column"). The coder was
including every column from model.column_names, so any model with a
t.virtual column would crash mount.

Filter columns by adapter capability + Column#virtual?, mirroring how
Rails' build_fixture_sql filters them in connection_adapters/abstract/
database_statements.rb.

Adds a computed_widgets table with a stored generated column to the
dummy app and a regression spec exercising the round-trip.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>