Add semantic selectors for iced tests

[caniko]

Summary

Adds semantic metadata to widget operations so iced_selector and iced_test can target widgets by role, label, and stable test id. This is intended to make headless E2E tests less dependent on visible text or implementation-only widget ids.

Changes

• Add iced_core::widget::metadata::{Metadata, Role}.
• Add a non-breaking Operation::metadata hook.
• Extend iced_selector candidates and targets with metadata.
• Add semantic selectors: by_role, by_label, by_role_and_label, and by_test_id.
• Add widget::Semantics / widget::semantics(...) for explicit metadata.
• Emit metadata from button, checkbox, text_input, scrollable, and text.
• Extend .ice instructions to accept semantic targets like test_id="profile-name" and role=button label="Create".

Validation

nix shell nixpkgs#cargo nixpkgs#rustfmt -c cargo fmt --check
nix shell nixpkgs#cargo nixpkgs#rustc nixpkgs#rustfmt nixpkgs#stdenv.cc -c cargo test -p iced_selector
nix shell nixpkgs#cargo nixpkgs#rustc nixpkgs#rustfmt nixpkgs#stdenv.cc -c cargo test -p iced_widget
nix shell nixpkgs#cargo nixpkgs#rustc nixpkgs#rustfmt nixpkgs#stdenv.cc -c cargo test -p iced_test
nix shell nixpkgs#cargo nixpkgs#rustc nixpkgs#rustfmt nixpkgs#clippy nixpkgs#stdenv.cc nixpkgs#pkg-config nixpkgs#openssl.dev -c bash -lc 'export PKG_CONFIG_PATH=$(nix eval --raw nixpkgs#openssl.dev.outPath)/lib/pkgconfig${PKG_CONFIG_PATH:+:$PKG_CONFIG_PATH}; cargo lint'