Implement mutable stylesheet CSSOM for Monaco and Code OSS #74

Problem

HTMLStyleElement.sheet currently returns a stable object containing only ownerNode. It lacks cssRules, insertRule, and deleteRule. Code OSS and Monaco add and remove runtime decoration/theme rules through this CSSOM surface. The authored CSS engine can already parse replacement style text and recascade, but JavaScript cannot perform rule mutation.

SceneTech/vscode-demo currently wraps the sheet object and republishes mutations through the owning style element's textContent. This proves the Code OSS path, but it is deliberately incomplete: nested grouping rules, imported/constructed sheets, native rule identity, and browser-accurate CSS text are outside that adapter.

WebScene #70 caches compiled CSS and introduces typed CSS IR. That work is complementary; CSSOM mutation must invalidate/reuse those caches correctly without storing document-specific cascade state.

Proposed implementation

• Add native CSSStyleSheet, CSSRuleList, CSSRule, and CSSStyleRule wrappers with stable object identity.
• Implement cssRules, insertRule, and deleteRule for connected style elements with CSSOM index/error semantics.
• Expose mutable CSSStyleDeclaration on style rules and route changes through the existing parser, stylesheet ownership, cascade invalidation, layout, and paint pipeline.
• Handle grouping rules and stylesheet replacement explicitly; define supported behavior for link, imported, and constructed sheets.
• Integrate cache invalidation with WebScene Compile and cache CSS units across runtime and native builds #70 if/when it lands.

Acceptance

• Code OSS editor decorations and theme rules render and can be removed without a host-injected stylesheet shim.
• Tests cover insertion order, deletion, live rule lists, stable rule identity, external textContent replacement, declaration mutation, disconnected owners, invalid indices/syntax, shadow-root ownership, nested rules, recascade, and cache invalidation.
• Top-level and iframe realms expose the same CSSOM behavior.

Reproduction

Use SceneTech/vscode-demo at c37b252e372dbf8fe04f912bfda0c37e0f3520d0, remove src/css_compat.hpp, build/package, and exercise Monaco decorations. style.sheet.insertRule is unavailable.