Implement lazy table header initialization to fix a bug where
empty table headers were printed when all rows were filtered out
during data-only diffs. This occurred because BeginTable() was
called before row filtering, causing headers to print even when
no matching rows existed.

The solution introduces a lazyRowWriter that delays the BeginTable()
call until the first row is actually written. This wrapper is only
used when:
- A filter is active (added/modified/removed)
- The diff is data-only (no schema changes or table renames)

Implementation changes:
- Add shouldUseLazyHeader() helper to determine when to use lazy
initialization based on filter presence and diff type
- Add lazyRowWriter type that wraps SqlRowDiffWriter and delays
BeginTable() until first WriteRow() or WriteCombinedRow() call
- Modify diffUserTable() to skip BeginTable when using lazy writer
- Modify diffRows() to conditionally create lazyRowWriter vs normal
rowWriter based on shouldUseLazyHeader() check
- Add comprehensive unit tests for shouldUseLazyHeader logic and
lazyRowWriter behavior (5 test functions, 8+ test cases)
- Add mock implementations of diffWriter and SqlRowDiffWriter
interfaces to enable testing without database dependencies
- Fix BATS test assertions to match actual SQL output format
(lowercase type names, MODIFY COLUMN vs DROP/ADD pattern)

Test coverage:
- TestShouldUseLazyHeader: validates lazy header logic conditions
- TestLazyRowWriter_NoRowsWritten: verifies BeginTable not called
when no rows written (core lazy behavior)
- TestLazyRowWriter_RowsWritten: verifies BeginTable called on
first write
- TestLazyRowWriter_CombinedRowsWritten: tests combined row writes
- TestLazyRowWriter_InitializedOnlyOnce: ensures BeginTable called
exactly once

Refs: dolthub#1430

Extract duplicated row filter checking logic into a reusable
shouldSkipDiffType helper function.

Refs: dolthub#1430

Refactor lazyRowWriter to use a cleaner callback-based
architecture that reduces struct complexity from 8 fields to 2
fields. This addresses PR review feedback requesting
simplification of the lazy header implementation.

Changes:
- Replace parameter storage with closure-based callback pattern
that captures BeginTable parameters from outer scope
- Eliminate separate ensureInitialized() method in favor of
inline initialization check in WriteRow() and
WriteCombinedRow()
- Remove initialized bool field by using callback presence (nil
check) to track initialization state
- Always create RowWriter upfront and only delay BeginTable
call, eliminating the need for writer factory function
- Simplify Close() method to always delegate to wrapped writer

Refs: dolthub#1430

Address @elianddb feedback on PR dolthub#10030 by standardizing DiffType
usage, simplifying filter logic, and fixing row-level filtering.

Changes:
- Fix row filtering bug: add early return for ChangeType.None to
prevent incorrectly skipping added/removed rows
- Standardize DiffType: replace string literals with constants
(DiffTypeAdded/Modified/Removed) in table_deltas.go and merge.go
- Simplify table filtering: use delta.DiffType directly
- Update tests: rewrite to use map-based architecture and
newDiffTypeFilter() constructor

Refs: dolthub#1430

Add DiffTypeRenamed constant and "removed" as user-friendly alias
for "dropped" filter option. This provides more granular filtering
and improves user experience with familiar terminology.

List of changes:
- Add DiffTypeRenamed constant for renamed tables
- Revert GetSummary to use DiffTypeRenamed instead of treating
renames as modified (table_deltas.go:742)
- Add "removed" as alias that maps to "dropped" internally for
user convenience
- Update filter validation to include renamed option
- Update CLI help text to document all filter options including
renamed and removed alias
- Handle renamed tables in merge stats alongside modified
- Update getDiffSummariesBetweenRefs and
getSchemaDiffSummariesBetweenRefs to handle renamed diff type
- Update all tests to use new constants and test renamed
filtering

Filter options now available: added, modified, renamed, dropped,
removed (alias for dropped).

Refs: dolthub#1430

Add tests for the --filter=renamed option and the --filter=removed alias
that maps to dropped.

Go tests:
- Tests for filter=renamed checking all diff types
- Tests for "removed" alias mapping to dropped internally
- Verify renamed filter only includes renamed tables

BATS tests:
- Test --filter=renamed with table rename scenario
- Test --filter=dropped with table drop scenario
- Verify --filter=removed alias works same as dropped
- Verify other filters correctly exclude renamed/dropped tables

Refs: dolthub#1430

…e/1430/add-filter-opt-for-diff

Update the short help text for the --filter parameter to document
all valid filter options (added, modified, renamed, dropped) and
mention that 'removed' is accepted as an alias for 'dropped'.

Refs: dolthub#1430