Bump default dev Scala to 3.6.4 and keep 3.4.1 in cross builds

[jonathanvdc]

Motivation

• Raise the default developer/IDE Scala version to 3.6.4 while retaining 3.4.1 in the cross build matrix so older Scala 3 compatibility is still tested.

Description

• Update devScala in build.sbt from "3.4.1" to "3.6.4" and add "3.4.1" explicitly into ThisBuild / crossScalaVersions so cross builds include both 3.6.4 and 3.4.1 along with the existing 2.x versions.

Testing

• No automated tests were run as part of this change.

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Codex Task

[github-actions]

JMH comparison (baseline vs PR)

Benchmark	Params	Baseline	PR	Δ (PR/Base)	Unit
IncrementalBenchmarks.incrementalPolynomial	depth=6, mutableEGraph=false, size=500, threadCount=1	196.111	198.493	1.012× (+1.2%)	ms/op
IncrementalBenchmarks.incrementalPolynomial	depth=6, mutableEGraph=false, size=500, threadCount=2	339.354	337.386	0.994× (-0.6%)	ms/op
IncrementalBenchmarks.incrementalPolynomial	depth=6, mutableEGraph=true, size=500, threadCount=1	8.50262	8.23155	0.968× (-3.2%)	ms/op
IncrementalBenchmarks.incrementalPolynomial	depth=6, mutableEGraph=true, size=500, threadCount=2	89.6414	90.1259	1.005× (+0.5%)	ms/op
IncrementalBenchmarks.oneByOnePolynomial	depth=6, mutableEGraph=false, size=500, threadCount=1	324.915	340.776	1.049× (+4.9%)	ms/op
IncrementalBenchmarks.oneByOnePolynomial	depth=6, mutableEGraph=false, size=500, threadCount=2	562.13	561.223	0.998× (-0.2%)	ms/op
IncrementalBenchmarks.oneByOnePolynomial	depth=6, mutableEGraph=true, size=500, threadCount=1	140.067	142.925	1.020× (+2.0%)	ms/op
IncrementalBenchmarks.oneByOnePolynomial	depth=6, mutableEGraph=true, size=500, threadCount=2	343.037	344.775	1.005× (+0.5%)	ms/op
LiarBenchmarks.findGemmInMm	threadCount=1	221.996	239.649	1.080× (+8.0%)	ms/op
LiarBenchmarks.findGemmInMm	threadCount=2	174.968	184.193	1.053× (+5.3%)	ms/op
LiarBenchmarks.findGemvInMv	threadCount=1	63.5265	67.69	1.066× (+6.6%)	ms/op
LiarBenchmarks.findGemvInMv	threadCount=2	56.4987	58.7049	1.039× (+3.9%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=false, size=20, threadCount=1	4.32065	4.55437	1.054× (+5.4%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=false, size=20, threadCount=2	4.40878	4.54582	1.031× (+3.1%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=false, size=40, threadCount=1	31.2405	30.899	0.989× (-1.1%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=false, size=40, threadCount=2	24.262	23.822	0.982× (-1.8%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=false, size=80, threadCount=1	261.478	252.564	0.966× (-3.4%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=false, size=80, threadCount=2	163.233	165.366	1.013× (+1.3%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=true, size=20, threadCount=1	2.83741	2.86605	1.010× (+1.0%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=true, size=20, threadCount=2	2.81546	2.82661	1.004× (+0.4%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=true, size=40, threadCount=1	21.3237	20.5232	0.962× (-3.8%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=true, size=40, threadCount=2	16.599	15.9971	0.964× (-3.6%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=true, size=80, threadCount=1	168.336	170.827	1.015× (+1.5%)	ms/op
MatmulBenchmarks.nmm	mutableEGraph=true, size=80, threadCount=2	105.918	102.642	0.969× (-3.1%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=false, size=5, threadCount=1	87.4911	87.3936	0.999× (-0.1%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=false, size=5, threadCount=2	82.9019	81.6584	0.985× (-1.5%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=false, size=6, threadCount=1	407.816	404.838	0.993× (-0.7%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=false, size=6, threadCount=2	366.353	359.806	0.982× (-1.8%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=true, size=5, threadCount=1	34.8101	34.3601	0.987× (-1.3%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=true, size=5, threadCount=2	29.8796	29.4649	0.986× (-1.4%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=true, size=6, threadCount=1	143.858	139.53	0.970× (-3.0%)	ms/op
PolyBenchmarks.polynomial	mutableEGraph=true, size=6, threadCount=2	116.613	113.323	0.972× (-2.8%)	ms/op
VectorBenchmarks.blinnPhong	mutableEGraph=false, threadCount=1	34782.8	34755.6	0.999× (-0.1%)	ms/op
VectorBenchmarks.blinnPhong	mutableEGraph=false, threadCount=2	32625.3	32955.3	1.010× (+1.0%)	ms/op
VectorBenchmarks.blinnPhong	mutableEGraph=true, threadCount=1	4187.54	4175.06	0.997× (-0.3%)	ms/op
VectorBenchmarks.blinnPhong	mutableEGraph=true, threadCount=2	3138.78	3037.42	0.968× (-3.2%)	ms/op
VectorBenchmarks.gramSchmidt	mutableEGraph=false, threadCount=1	472.821	470.694	0.996× (-0.4%)	ms/op
VectorBenchmarks.gramSchmidt	mutableEGraph=false, threadCount=2	388.527	388.144	0.999× (-0.1%)	ms/op
VectorBenchmarks.gramSchmidt	mutableEGraph=true, threadCount=1	171.399	168.683	0.984× (-1.6%)	ms/op
VectorBenchmarks.gramSchmidt	mutableEGraph=true, threadCount=2	131.751	126.975	0.964× (-3.6%)	ms/op
VectorBenchmarks.reflection	mutableEGraph=false, threadCount=1	292.997	298.635	1.019× (+1.9%)	ms/op
VectorBenchmarks.reflection	mutableEGraph=false, threadCount=2	246.864	242.987	0.984× (-1.6%)	ms/op
VectorBenchmarks.reflection	mutableEGraph=true, threadCount=1	125.587	126.182	1.005× (+0.5%)	ms/op
VectorBenchmarks.reflection	mutableEGraph=true, threadCount=2	92.836	91.3224	0.984× (-1.6%)	ms/op
VectorBenchmarks.vectorNormalization	mutableEGraph=false, threadCount=1	3.76905	3.75948	0.997× (-0.3%)	ms/op
VectorBenchmarks.vectorNormalization	mutableEGraph=false, threadCount=2	4.51072	4.48549	0.994× (-0.6%)	ms/op
VectorBenchmarks.vectorNormalization	mutableEGraph=true, threadCount=1	2.07052	1.74756	0.844× (-15.6%)	ms/op
VectorBenchmarks.vectorNormalization	mutableEGraph=true, threadCount=2	2.46763	2.4995	1.013× (+1.3%)	ms/op
Geomean	threadCount=1, mutableEGraph=false	—	—	1.016× (+1.6%)
Geomean	threadCount=1, mutableEGraph=true	—	—	0.977× (-2.3%)
Geomean	threadCount=2, mutableEGraph=false	—	—	1.005× (+0.5%)
Geomean	threadCount=2, mutableEGraph=true	—	—	0.985× (-1.5%)

Note: < 1.0× means faster on PR; > 1.0× means slower.