Keep BlockBuilder restart counter valid after ChangeOptions

[fyrsta7]

Summary

Fixes #1339 by keeping BlockBuilder's restart counter consistent when TableBuilder::ChangeOptions lowers block_restart_interval after entries have already been added.

Details

BlockBuilder::counter_ tracks how many entries have been emitted since the last restart point. If ChangeOptions installs a smaller restart interval while counter_ is already larger than that interval, the next Add can violate the builder's restart interval invariant.

This adds an OnOptionsChanged hook that clamps the counter to the current interval after the table builder updates its options. The new regression test builds a table block under interval 16, changes the interval to 2, adds another entry, finishes and reopens the table, and verifies that the entry can be found through an iterator.

Validation

I built and ran the repository's complete leveldb_tests binary. Of 212 tests, 211 passed and one compression-dependent test was skipped by the current build configuration; there were no failures.

I also ran the new regression test and the full TableTest.* group separately:

./build-pr-submodules/leveldb_tests \
  --gtest_filter=TableTest.ChangeOptionsLowerRestartInterval
./build-pr-submodules/leveldb_tests --gtest_filter=TableTest.*

git diff --check also passes.