Refactor classNames scanner and add classList

[jhy]

Improve throughput and allocation behavior in classNames() by replacing the regex split path with a small HTML-whitespace scanner.

Add classList() as an immutable snapshot API for callers that only need to read class names, with a more efficient empty result path for elements without classes.

Align classNames(), classList(), and hasClass() on the HTML whitespace definition for class attributes.

Benchmarks

classNames() Throughput:

Fixture	Scope	1.22.2	New	Gain
tiny	all elements	36.0M ops/s	270.6M ops/s	652.1%
small	all elements	39.1M ops/s	153.3M ops/s	291.6%
medium	all elements	28.3M ops/s	144.5M ops/s	411.3%
large	all elements	30.1M ops/s	63.2M ops/s	110.3%
x-large	all elements	11.7M ops/s	22.1M ops/s	88.9%
small	classed elements	35.1M ops/s	54.6M ops/s	55.3%
medium	classed elements	18.3M ops/s	43.6M ops/s	137.8%
large	classed elements	21.2M ops/s	26.2M ops/s	23.6%
x-large	classed elements	9.7M ops/s	15.2M ops/s	56.8%

classNames() Allocation:

Fixture	Scope	1.22.2	New	Saving
tiny	all elements	376 B/op	80.0 B/op	78.7%
small	all elements	352 B/op	89.7 B/op	74.5%
medium	all elements	460 B/op	93.1 B/op	79.8%
large	all elements	376 B/op	104 B/op	72.3%
x-large	all elements	544 B/op	203 B/op	62.7%
small	classed elements	376 B/op	152 B/op	59.6%
medium	classed elements	511 B/op	171 B/op	66.6%
large	classed elements	376 B/op	152 B/op	59.6%
x-large	classed elements	560 B/op	261 B/op	53.4%

classList() Baseline:

Fixture	Scope	classList() throughput	classList() allocation
tiny	all elements	1,355.8M ops/s	~0 B/op
small	all elements	285.3M ops/s	3.2 B/op
medium	all elements	215.6M ops/s	6.4 B/op
large	all elements	112.4M ops/s	8.1 B/op
x-large	all elements	26.1M ops/s	79.0 B/op
small	classed elements	87.0M ops/s	24.0 B/op
medium	classed elements	61.0M ops/s	44.8 B/op
large	classed elements	40.2M ops/s	24.0 B/op
x-large	classed elements	18.6M ops/s	117 B/op

Fixture Class Histogram:

Fixture	Input	Elements	[class] elements	0 classes	1 class	2 classes	3 classes	4+ classes	Avg classes / [class]
tiny	274 B	11	0	11	0	0	0	0	0.00
small	3.3 KB	45	6	39	6	0	0	0	1.00
medium	8.6 KB	160	22	138	19	3	0	0	1.14
large	496.3 KB	6,208	2,145	4,063	2,145	0	0	0	1.00
x-large	2.3 MB	10,310	7,347	2,963	3,435	1,594	1,245	1,073	2.20

(Single threaded performance, relative comparison)