Prefetch join hash table bucket

[IsaacWarren]

Summary

• Adds a prefetch_bucket method to the vendored ankerl hash map that issues a read-only __builtin_prefetch for the bucket cache line find() will touch
• Adds a software-prefetch pass to all 4 join probe loops: while probing row i, the bucket line for row i + D (D=16, tunable via BODO_JOIN_PROBE_PREFETCH_DIST) is prefetched, hiding part of the dependent cache-miss chain that dominates ht_probe_time
• Hoists three per-row pin-guard resolutions in produce_probe_output out of the loop (resolved once per batch, passed as raw pointers)

The join probe hot loop is a pointer chase: hash -> bucket line -> entry/key line. Hardware prefetchers cannot help (non-stride). ht_probe_time was ~30% of the probe stage, the largest join-side timer.

Performance findings

All benchmarks: 8 workers, TPC-H SF10, interleaved A/B rounds, stage metrics from BODO_TRACING_OUTPUT_DIR (wall-clock medians; medians of 3 runs for the microbench).

Pure join microbenchmark (lineitem 60M x orders 15M int keys)

prefetch distance	ht_probe_time / rank-exec	delta
off	0.457 / 0.443s	-
8	0.319 / 0.299s	-31%
16 (default)	0.304 / 0.293s	-34%
32	0.303 / 0.303s	-33%

Probe stage total -13%.

Full TPC-H SF10, all 22 queries

metric	off	on (D=16)	delta
wall, sum of per-query best	92.74s	82.98s	-10.5%
ht_probe_time	0.347s	0.218s	-37%
probe stage total	1.017s	0.835s	-18%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.16%. Comparing base (0f7e335) to head (9cb0fd3).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1390      +/-   ##
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+ Coverage   66.55%   68.16%   +1.61%     
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  Files         198      198              
  Lines       68731    68961     +230     
  Branches     9834     9905      +71     
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+ Hits        45744    47009    +1265     
+ Misses      20071    19077     -994     
+ Partials     2916     2875      -41     

[DrTodd13]

LGTM. thanks