fix(runtime): preserve full-mode Markdown contrast

[xiaotiepen]

Summary / 摘要

• Extend the wide task-page Markdown color override to taskMode=full, so explicit light themes do not inherit the native dark-shell white text.
• Apply the existing light-shell readability shadow to the same full state without changing artwork, veil strength, or theme-package contents.
• Sync the canonical runtime CSS into both platform assets and add a shared renderer regression assertion.
• Update the macOS and Windows changelogs.

Closes #309

Type / 类型

• Bug fix / 缺陷修复
• Feature / 新功能
• Docs / 文档
• Theme / CSS / visual / 主题或视觉
• Scripts / install / restore / 脚本或安装恢复
• Chore / 杂项

Platform / 平台

• macOS
• Windows
• Both / 双平台
• Docs / repo only / 仅文档或仓库元数据

Self-check / 自测

Docs-only / 仅文档

• Links and wording reviewed / 已检查链接与表述

macOS (when code under macos/ changes)

• macos/tests/run-tests.sh passed / 已通过
• Doctor (optional): macos/scripts/doctor-macos.sh
• Live verify (if inject/CSS/start path): verify-dream-skin-macos.sh or Desktop Verify
• Restore / re-apply smoke (if install/restore/start changed) / 若改了安装恢复启动则做过恢复再应用

Windows (when code under windows/ changes)

• Relevant install / start / verify / restore scripts exercised / 已按改动跑过对应脚本
• Environment noted below (OS build, Codex source) / 下方注明环境

User-facing / 用户可见变更

• Updated macos/CHANGELOG.md (and macos/VERSION if release-worthy) / 已更新 changelog（发版时再 bump VERSION）
• N/A — no user-facing change / 无用户可见变更

Security / 安全

• Does not modify official Codex install / asar / signatures / 未修改官方安装与签名
• Does not silently write API Base URL or keys / 未静默写入 API Base URL 或 Key
• CDP remains loopback-oriented (127.0.0.1) where applicable / CDP 仍仅本机回环（如适用）

Notes / 补充

• Reproduced on macOS 26.5.2 arm64, Codex 26.721.41059, and Dream Skin 1.5.6 with an explicit light, wide taskMode=full theme. Before the patch, visible Markdown computed to rgb(255, 255, 255) while --ds-text was #1a1b1e; the equivalent runtime patch computes to rgb(26, 27, 30) and leaves the full-mode veil unchanged.
• Passed: node tools/sync-runtime-assets.mjs --check, macOS and Windows renderer runtime tests, both platform payload checks, nested-:has() checks, and git diff --check.
• Passed the complete portable macOS suite with CODEX_DREAM_SKIN_SKIP_SIGNED_RUNTIME_TESTS=1 CODEX_DREAM_SKIN_SKIP_DOCTOR=1 bash macos/tests/run-tests.sh. Signed installed-runtime integration and Doctor were skipped because this host's installed 1.5.6 engine contains the equivalent local hotfix; native SwiftPM/XCTest was skipped by the suite because a full matching Xcode platform is unavailable.
• Windows native UI was not available; Windows coverage is the byte-identical generated CSS plus the Windows renderer and payload checks. No install/start/restore code changed.