perf(join): fall back to chained build on pathological key duplication

[singaraiona]

Summary

Fixes the O(dup²) build pathology in the radix-parallel join at its source. The per-partition open-addressing hash build is row-granular, so a key with dup duplicate rows forms a dup-length linear-probe run — inserting the k-th scans k slots → quadratic build (a 10M single-key build took ~9.8s). The build now counts same-hash (per-key) duplicates during insert and, when one key exceeds RADIX_DUP_RUN_MAX (512), abandons the radix attempt and re-runs the whole join through the existing chained-HT path (O(1) insert / O(matches) probe — already the trusted reference for small joins and OOM recovery).

This is sub-project 4, piece 2. It complements the merged build-side selection (PR #251): that dodges the quadratic for INNER joins by building the smaller side; this fix covers LEFT/FULL (which can't swap) and any forced-build-duplicated-side case. ANTI joins use a separate exec path and are unaffected.

Perf (gate verdict: WIN)

• Catastrophic (one giant build key): ~16× — INNER-no-swap and LEFT both ~2200ms → ~135ms.
• Moderate (~100 rows/key): neutral, no trip — the same-hash counter measures true per-key duplication, immune to the dense-cluster collision-merge that a total-run-length signal (and lowering the load factor) could not separate.
• Near-unique + ~10/key: neutral — the per-insert branchless count + single post-loop check costs nothing (a loop-cloning codegen trap from reading the knob in the hot loop was found and avoided).

Full data in `bench/bottleneck/join_dup_fallback_compare.md`.

Test Plan

• Differential (`test_join_buildside.c`): auto-fallback == forced-chained-oracle multiset for INNER/LEFT/FULL; trip-boundary, not-sticky, no-trip control, force-on-ordinary
• Correctness inherited from the chained path; no partial radix output leaks on a trip (cleanup runs before consolidation)
• Full suite green under ASan+UBSan (3451/3453, 2 pre-existing skips); both abort points (mid-build goto, top-of-fn force bail) exercised
• Release build warning-free; bench binary sanitizer-free