Support Datastar v1.0.1 data-bind __prop/__event and data-on __document modifiers

[maragubot]

Summary

Adds support for the new attribute modifiers introduced in Datastar v1.0.0 and refined in v1.0.1:

• data-bind __prop modifier — bind through a specific property (camelCased at runtime in v1.0.1)
• data-bind __event modifier — define which events sync the element back to the signal
• data-on __document modifier — attach event listeners to the document element

In v1.0.1 the __prop and __event modifiers may be used independently of each other, so both the single- and combined-modifier forms are valid.

API additions

• Prop(name string) Modifier — returns the full __prop.name modifier. Panics on empty name.
• Event(names ...string) Modifier — returns the full __event.a.b.c modifier. Panics when no names or any empty name is given.
• ModifierDocument constant
• Bind() now accepts modifiers ...Modifier (source-compatible with existing single-argument calls; all demo/main.go call sites unchanged)

Usage

data.Bind("isChecked", data.Prop("checked"))
data.Bind("query", data.Event("input", "change"))
data.Bind("isChecked", data.Prop("checked"), data.Event("change"))
data.On("keydown", "$foo = ''", data.ModifierDocument)

Other

• Bump vendored docs/datastar.js to v1.0.1
• Update AGENTS.md compatibility note to v1.0.1

Runtime-only v1.0.0/v1.0.1 changes (morphing, fetch behavior, kebab fix, select signal-type fix, retryMaxWaitMs rename, Rocket rewrite) are out of scope for this library.

Test plan

• make test passes (95.6% coverage)
• Tests cover __prop, __event (single and multiple events), combined __prop+__event, and __document
• Panic tests for Prop(""), Event(), and Event("x", "")
• Example functions added for godoc
• Vendored bundle verified to no longer contain the v1.0.0 BindPropRequiredWhenEventProvided / BindEventRequired pairing constraint