refactor(storybook-builder-parcel): align iframe & preview entry with upstream
After externalizing storybook runtime modules (Task 1.1) and hoisting
the runtime import (Task 1.2), runtime.js's setup() populates every
__STORYBOOK_MODULE_* global plus __STORYBOOK_ADDONS_PREVIEW and
__STORYBOOK_ADDONS_CHANNEL__. Our hand-rolled init-addons IIFE and
preview.js bootstrap entry exist only to mask the cycle that those
populate-the-globals tasks now resolve at the source.
- iframe.html: drop the inline init IIFE, add TAGS_OPTIONS / OTHER_GLOBALS
placeholders, set window.module=undefined and window.global=window
(matches upstream Vite); script tag now points at preview-main.js
- gen-preview-modern.mjs: remove generatePreviewBootstrap and
generateInitAddonsGlobal helpers and their exports
- preset.mjs: stop emitting preview.js and init-addons-global.js to
generated-entries; only setup-addons.js and preview-main.js are needed
The patch (patches/storybook+10.3.5.patch) is still in place and
removed in Task 1.4.