htmx 4: new core attributes and hx-partial element

[maragubot]

Part of #18. Depends on the Modifier type from the modifiers sub-issue. Core attributes only; extension attributes are out of scope by decision. Reference: https://four.htmx.org/reference/attributes

New attribute helpers

Helper	Attribute	Notes
Action(url)	hx-action	URL for the request, mirroring native action.
Method(v)	hx-method	get, post, put, patch, delete, query.
Query(url)	hx-query	Issues an HTTP QUERY request.
Config(v)	hx-config	HCON or JSON string. Replaces hx-request.
Status(code, v)	hx-status:<code>	Key-form attribute like On. code is a string so 50x/5xx wildcards work.
MorphSkip()	hx-morph-skip	Bare attribute.
MorphSkipChildren()	hx-morph-skip-children	Bare attribute.

All take ...Modifier. Values stay plain strings; typed/map forms for JSON-valued attributes are a possible follow-up, not part of this.

New element: hx-partial

<hx-partial hx-target="..." hx-swap="..."> is a response-side element for multi-target updates. https://four.htmx.org/reference/tags/hx-partial

func Partial(children ...g.Node) g.Node  // g.El("hx-partial", children...)

Targets and swaps are passed as ordinary Target/Swap attribute children. The <template hx type="partial"> variant is not needed since gomponents does not strip unknown tags.

Acceptance criteria

• Each helper above exists with a verbatim doc comment linking to the reference page, a test, and a godoc Example.
• Partial renders <hx-partial hx-target="#x" hx-swap="outerHTML">...</hx-partial>.