MDEV-38284: Add configurable ef_construction for MHNSW vector index

[MauricioPerera]

Summary

Add per-index ef_construction parameter to control the number of candidates evaluated during HNSW graph construction. Higher values create more accurate index graphs at the cost of slower inserts.

This is a focused PR that only adds ef_construction configurability (split from #4589 based on reviewer feedback to separate features).

Benchmark Results (SIFT dataset, 50K vectors, 128 dims)

ef_construction	ef_search	recall@10	query_time
10 (current)	10	99.75%	20.95ms
100	10	100%	9.81ms

Key findings:

• 2.1x faster queries with ef_construction=100 at same ef_search
• Build time: 1.6x slower (559s vs 353s)
• Break-even: ~18,500 queries

Use Case

For workloads where the index is built once but queried millions of times, higher ef_construction saves significant total time by enabling faster queries at the same recall level.

Changes

• Replace static constexpr ef_construction=10 with per-index option
• Add mhnsw_default_ef_construction system variable (range 1-10000, default 10)
• Add ef_construction to ha_index_option_struct and MHNSW_Share
• Add HA_IOPTION_SYSVAR for ef_construction in mhnsw_index_options
• Add test for ef_construction parameter

Usage

-- Via system variable (affects new indexes)
SET mhnsw_default_ef_construction = 100;
CREATE TABLE t (v VECTOR(128) NOT NULL, VECTOR INDEX (v));

-- Via index option
CREATE TABLE t (v VECTOR(128) NOT NULL, VECTOR INDEX (v) ef_construction=100);

-- Combined with M parameter
CREATE TABLE t (v VECTOR(128) NOT NULL, VECTOR INDEX (v) M=16 ef_construction=100);

🤖 Generated with Claude Code

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[gkodinov]

Thank you for your contribution. This is a preliminary review.
Please manage the contribution license agreement.
There are some failures of the new test on certain platforms. Please fix this too.

[gkodinov]

it also feels like we need a new MDEV for this. Can you please open one with a good description?