Summary

Fixes #15.

This change keeps the cached update bind plan for unfiltered updates, but builds
filtered update plans per call. ColumnFilter functions are supplied by callers
and do not have a stable cache key, so reusing one table-level cached plan across
different filters can make UpdateColumns write columns rejected by the current
filter.

The new regression coverage checks both full-to-filtered and filtered-to-filtered
update sequences, including the exact generated SQL and bound values.

Verification

• go test ./... -run '^(TestUpdateColumnsDoesNotReusePriorFullUpdatePlan|TestUpdateColumnsDoesNotReusePriorFilteredUpdatePlan|TestMySQLDialect|TestPostgresDialect)$' -count=1 -v
• go test ./... -count=1
• go build ./...
• git diff --cached --check