Keep the parts of the old compaction work that still help

[venkat1701]

#73 to #76 sped up the old compaction, which walked the trees and rewrote the paths to moved pages. #81 replaced that with moving pages through the page directory and counting references, so most of that stack no longer applies and it is being closed. This PR keeps the three pieces that still help on the current code.

What changes

• Inline posting lists are decoded without streams (from Walk trees for compaction without allocating per page #75). Each inline list used to go through a stream, a list that allows nulls and a second copy. It is now built once with List.of.
• Nested trees are visited without rebuilding records (from Walk trees for compaction without allocating per page #75, adapted). mapRefs built a new record for every leaf value even when every reference mapped to itself. It now returns the same record in that case, and a new forEachRef visits the references without building anything. The reference counter's walk (TreeWalker.references) and the check that sets a leaf's REFERS_TO_PAGES flag now use forEachRef, so the counter no longer builds one record per edge or promoted posting list it passes.
• A crash test for checkpoints that overlap commits (from Keep commits fast while compaction runs #76). A writer commits while the test checkpoints every 20 ms, the engine stops without a clean shutdown, and every commit has to come back with the counts matching the trees.

Not carried over

#76 also moved the catalog write and the WAL sync of a checkpoint out of the commit lock. #81 already syncs the segments before taking the lock, but the catalog publish and WAL sync still run under it. While writing the test I saw what that costs: with checkpoints running back to back, a writer gets very few commits in. Doing that part again on top of the reference count journal is a separate change.

Testing

• Full suite: 155 tests pass.
• New AtomCodecTest and PostingsCodecTest check that mapRefs keeps the same record when nothing changes and that forEachRef sees the same references mapRefs does.
• CrashRecoveryTest.checkpointsTakenWhileCommitsRunLoseNothing runs in both WAL modes, about 13 s together.