fix: preserve schema order when adding columns

[KKould]

Summary

• keep newly added column ids strictly after all existing column ids
• prevent schema reload from reordering columns after ALTER TABLE ADD COLUMN
• add a regression test covering the column id ordering invariant

Why

The flaky test_model_migrate failure came from schema order drifting from id,name,age to id,age,name after ADD COLUMN, which made tuple decoding read the row with the wrong layout and triggered UnexpectedEof.

Verification

• cargo test test_add_column_generates_id_after_existing_columns -- --nocapture
• cargo test -p macros-test test_model_migrate -- --nocapture