Fix table validation bugs

[0x00-sys]

• select_to() was documented but only contained ..., so it returned None and inserted nothing. It now inserts projected rows through target.insert() so normal validation still applies.
• Table schemas could be changed after construction by mutating the original column list or returned column objects. Tables now copy column definitions on input and output.
• INT accepted True and DATE accepted datetime because of Python subclassing. Validation now requires exact types for those fields.
• test_05_29.py failed pytest collection because NanoDB symbols were not imported. Added the imports and regression tests.