feat(wasm): add CreateWasmFuncWithReturn for value-returning JS callbacks

[felipegenef]

What's new

CreateWasmFuncWithReturn — Value-Returning JS Callback Constructor

Gothic WASM code can now hand a value-returning Go closure to JavaScript without
dropping down to ExecJS. CreateWasmFuncWithReturn wraps syscall/js.FuncOf, registers the
closure as a named global, and returns a JSValue holding the underlying js.Func
so it can be assigned directly to a JS object property.

This closes the last gap in the Go→JS callback family. Until now we had:

• CreateWasmFunc / CreateWasmStringFunc / CreateWasmBoolFunc — fire-and-forget
  callbacks invoked from HTML attributes, all returning void.

Third-party JS libraries (uPlot, Chart.js, Leaflet, ECharts, …) routinely accept
option callbacks that must return a value synchronously — axis tick formatters,
tooltip renderers, point-style resolvers. Those could not be expressed with the
void-returning helpers, forcing an ExecJS string-eval workaround that lost type
safety and Go-side state. CreateWasmFuncWithReturn makes them first-class.

The js.Func is retained for the lifetime of the page (added to the runtime keep
slice and never released), matching the persistence semantics of the existing
AddEventListener* and CreateWasm* helpers — so it is safe to install once during
ClientSideState setup.

Signature

func CreateWasmFuncWithReturn(name string, fn func(this JSValue, args []JSValue) any) JSValue

The returned any is marshalled back to JS via the same conversion used by
JSValue.Set/Call, so returning a JSValue, a string, a number, or a bool all work
as expected.

Example

import . "github.com/felipegenef/gothicframework/v2/pkg/wasm"

func ClientSideState() {
    // Register a callback and pass it as a property on a JS library config object.
    cb := CreateWasmFuncWithReturn("myFormatter", func(this JSValue, args []JSValue) any {
        return args[0].String() + "_suffix"
    })

    config := JS().Get("Object").New()
    config.Set("formatter", cb) // JS calls this and reads the return value
}

Bug fixes / improvements

None.

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