[#309] Add MVCC diagnostics to TreeTransactionalLifetimeTest.createRemoveByStep

[vharseko]

Refs #309. This PR does not close the issue: it adds diagnostics only.

Problem

TreeTransactionalLifetimeTest.createRemoveByStep still fails intermittently on CI (ubuntu-latest / JDK 21):

createRemoveByStep:182 -> createRemoveByStepHelper:215
expected:<0[],1:,2:a=step2,3,4:b=...> but was:<0[:],1:,2:a=step2,3,4:b=...>

Line 215 is the assert that runs inside the still-open ttlt5cr transaction, before its crash. A tree created at step 1 is reported visible at step 0 of the same transaction, while steps 1-4 are all correct (including "not visible" at step 3, after removeTree()). This is the same signature as #260 / #264. #266 (MVCC settle at the end of Persistit.initialize()) is already on master, so that explanation is incomplete. ttlt5cr is also a fresh tree name that nothing from the earlier crashed cases refers to.

Reading the code did not reveal the mechanism, and reproduction failed:

• macOS / JDK 21: in-JVM stress loop, 1000 cases, 0 failures;
• Linux / JDK 21 containers, 3 in parallel for CPU contention, a fresh JVM per run: 126 runs, 0 failures.

Change

Test-only. The assert is unchanged. On an expected1 mismatch the failure message now also carries:

• the transaction status and start timestamp;
• the tree's TimelyResource version chain after each step and at the failure. It is read reflectively, because Tree#toString() calls version(), which can add a version and change the state being reported;
• the Tree instance getTree() returns at each step (same object or a different one);
• the active transaction cache floor/ceiling and the non-empty TransactionIndex buckets.

The next CI occurrence will then show which entry makes the tree visible at step 0, which is what a product fix needs.

Verification

• TreeTransactionalLifetimeTest: 5/5 green (JDK 21).
• The diagnostic path was exercised by temporarily changing ttlt5cr's expected1. The message showed the expected chain (ts=…#01, #03 deleted, #04) and the per-step getTree results. The change was reverted afterwards.