…taSQL

getDown() previously called getOldValue() unconditionally, which crashes
with a fatal error when the diff contains DiffOpAdd objects (which lack
that method). Also, getDown() did not null-guard getOldValue(), causing
NULL column values to be written as empty strings instead of SQL NULL.

Both getUp() and getDown() now:
- Check for DiffOpAdd/DiffOpRemove to avoid calling missing methods
- Null-guard return values to emit col = NULL instead of col = ''

Added comprehensive unit tests covering DiffOpChange, DiffOpAdd,
DiffOpRemove, null values, and mixed DiffOp types in a single UPDATE.

Refs: bugs.md Bug #2, Bug #9, PR #93

Add a Fix Status summary table at the top of bugs.md documenting:
- Commit hash for each bug fix
- Brief description of what was changed
- Test file references
- Note that bugs #2 and #9 were combined (same file/method)

Original bug detail table preserved below for reference.