Fix routine diff handling and update docs

[bitner]

This PR improves makemigration for routine changes:

• emits CREATE OR REPLACE SQL for body-only function/procedure updates that results.schemadiff_as_sql() can miss
• avoids suppressing unsafe routine drops unless the same routine kind and signature still exist in the target schema
• adds regression coverage for the diff helpers and routine body-only changes
• updates README, CLI docs, architecture notes, and changelog

Validation:

• uv run pytest -q tests/unit tests/integration/test_diff.py tests/integration/test_executor.py tests/integration/test_tracking.py tests/integration/test_wrapper_end_to_end.py