seqlock-fix-4: the optimistic descent must not dereference a null child

[swapnilpaliwal-sd]

Closes #1869 — the parallel-solve crash is root-caused, captured, fixed and validated against a live reproduction.

The capture (lldb, java -j8, darwin-arm64, under a souffle -g memory-load loop — the missing repro ingredient):

thread #5, EXC_BAD_ACCESS (code=1, address=0x8)
frame #0: souffle::detail::btree<...>::insert(...) + 704
->  ldapr  w8, [x8]        ; acquire-load of a seqlock version at null+8

insert's descent reads next = cur->getChild(idx) under a still-unvalidated optimistic lease and immediately dereferences it (next->lock.start_read()). A concurrent split can expose a null child slot; x86's ordering keeps the window closed, arm64 under memory pressure opens it. This explains every observation: javascript 3/10 and java 2/4 crash rates, a different rule each time, arm64-only, pressure-gated, serial always clean.

The fix (overlay seqlock-fix-4, engine-id salted): a null child is treated as a failed validation and the insert restarts — the same contract the descent already follows for torn reads. Applied to BTree.h and BTreeDelete.h. Defense in depth in the same overlay: the flyweight fetch tolerates speculative indices (static empty value) and generic record unpack returns zeroed storage rather than an empty record's null data().

Validation, 1.2M-LOC java subject, interleaved runs under the identical load loop:

engine	segfaults
fix-3 (unfixed), -j8	8/10
fix-4, -j8	0/10

and fix-4 serial AND parallel outputs are sorted-identical to the fix-3 serial reference across every relation. The engine-package stub test carries the new anchors (green), and build-engines.yml applies the same awk-extracted patch with per-file assertions — its awk anchor is also repaired (the heredoc's import line had drifted under it, which would have failed the next release build loudly).

With this in, the parallel-by-default gate from #1861 stands safely for 0.1.9.

[swapnilpaliwal-sd]

The javascript leg repeats the result: under the same interleaved load loop the unfixed engine segfaulted 3/10 while fix-4 ran 0/10, and the fix-4 parallel output is sorted-identical to the unfixed serial reference across every relation. Both crashing languages now validate the same way.