feat(ui): expose textSetTextAlignment API for Text widget

[lzxb]

Summary

Perry's Text widget currently has no way to set horizontal text alignment (equivalent to CSS text-align). The underlying platform APIs all support this, but Perry doesn't expose it.

Evidence

All three platforms have the capability but none is wired to a user-callable FFI function:

Platform	Underlying API	Used internally?	Exposed to user?
macOS	NSTextField.setAlignment:	✅ toast.rs, chart.rs, bottom_nav.rs	❌
iOS	UILabel.setTextAlignment:	✅ toast.rs	❌
Android	TextView.setGravity(Gravity.CENTER_HORIZONTAL)	✅ image.rs, qrcode.rs	❌

Verified by inspecting:

• crates/perry-ui-macos/src/lib_ffi/core_widgets.rs — all perry_ui_text_set_* functions listed, no alignment
• crates/perry-ui-ios/src/ffi/widgets_basic.rs — same
• crates/perry-ui-android/src/ — same

Proposed API

// perry/ui type declaration
export function textSetTextAlignment(widget: Widget, alignment: number): void;

Alignment values (follow NSTextAlignment / UITextAlignment conventions):

• 0 = left
• 1 = right
• 2 = center
• 4 = natural (follows locale direction)

Current Workaround

Without this API, centering text requires wrapping in an extra container:

// Workaround: wrap Text in a VStack with CenterX alignment
function centeredText(content: string, parentWidth: number): Widget {
    const t = Text(content);
    const col = VStack(0, [t]);
    stackSetAlignment(col, 9);        // NSLayoutAttribute.centerX
    widgetSetWidth(col, parentWidth); // required reference width
    return col;
}

This adds an extra layout layer for what should be a single property.

Implementation Notes

The pattern already exists in the codebase — Perry uses setTextAlignment internally. Adding the public binding would follow the same 3-step pattern as other text setters:

1. Rust impl (crates/perry-ui-macos/src/lib_ffi/core_widgets.rs):

#[no_mangle]
pub extern "C" fn perry_ui_text_set_text_alignment(handle: i64, alignment: i64) {
    if let Some(view) = get_widget(handle) {
        unsafe { let _: () = msg_send![&*view, setAlignment: alignment]; }
    }
}

2. Dispatch table (crates/perry-dispatch/src/ui_table.rs): add a MethodRow entry

3. JS codegen (crates/perry-codegen-js/src/emit/calls.rs): add the method name → runtime symbol mapping

Android would use setGravity with the appropriate gravity flags for horizontal alignment.