fix(pyscript): use MutationObserver to adopt styles into CodeMirror shadow roots

[xransum]

Summary

• Replace the broken Element.prototype.attachShadow monkey-patch with a MutationObserver that watches for .py-editor-input nodes being inserted into the DOM and adopts the shared CSSStyleSheet into the shadow root that PyScript has already attached by that point.
• Fix token class names in buildCSS(): CodeMirror 6 emits tok-* class names via @lezer/highlight's classHighlighter, not the legacy cm-* names from CodeMirror 5.
• Fix the SVG run button icon disappearing in dark mode by overriding PyScript's hardcoded fill="#464646" with fill: currentColor.

Root causes

The monkey-patch approach had two independent failure modes:

1. Wrong element: PyScript attaches the shadow root to an anonymous <div> inside .py-editor-input, not on .py-editor-input itself. The class check on this (the element calling attachShadow) never matched because that element has no classes.

2. Patch overwritten: PyScript's core.js also patches Element.prototype.attachShadow via queueMicrotask, which runs after our synchronous script and silently replaces our patch with its own.

Fix

PyScript builds the full editor tree off-DOM, calls attachShadow on the inner div, then appends the whole tree to the page. By the time the MutationObserver fires on the inserted .py-editor-box, host.shadowRoot already exists and can be adopted into immediately -- no race condition.