perf: replace arr_build_full ht_head memset with versioned bucket heads

[justinjoy]

Background

Issue #740 recorded a negative finding from PR #731: the CRDT perf rep stos bar is not caused by the per-row timestamp memset in col_join_append_pair. That memset compiles inline and should not be retried as the target.

The likely hot site remains the full arrangement rebuild path:

• wirelog/columnar/arrangement.c: arr_build_full() clears arr->ht_head with memset(arr->ht_head, 0xFF, nbuckets * sizeof(uint32_t)) before rebuilding chains.
• CRDT recursive strata can trigger repeated full arrangement rebuilds as the right-side IDB grows.

Proposed direction

Investigate replacing the full ht_head clear with versioned bucket heads, for example:

• Add a generation/version side array or tagged bucket-head representation to col_arrangement_t.
• Increment the arrangement build generation on full rebuild instead of clearing every bucket.
• Treat a bucket as empty when its stored generation differs from the current build generation.
• Preserve existing UINT32_MAX/chain semantics or migrate them carefully.

This likely changes col_arrangement_t layout, so update the layout sentinel in tests/test_arrangement.c intentionally if the design lands.

Acceptance criteria

• Do not touch or retry lazy timestamp memset in col_join_append_pair; do-not-retry: lazy timestamp memset in col_join_append_pair (wrong target) #740 established that as the wrong target.
• Correctness preserved for existing arrangement tests, including incremental update, cache reuse, LRU eviction, K-fusion isolation, and join arrangement tests.
• Add targeted tests for version wrap/clear fallback if generation counters can wrap.
• Benchmark CRDT W=1/W=8 before and after; show whether the libc memset bar moves or disappears.
• If the design regresses memory, document the tradeoff and gate it behind a measured win.

References

• Follow-up to do-not-retry: lazy timestamp memset in col_join_append_pair (wrong target) #740
• PR perf(CRDT): leading-key compactor cache + perf-suite gate #731, "What was attempted but did not ship"
• wirelog/columnar/arrangement.c::arr_build_full
• wirelog/columnar/columnar_nanoarrow.h::col_arrangement_t
• tests/test_arrangement.c layout sentinel