Expose preferred text scale as a CSS environment value

[lanyue-llk]

Expose env(preferred-text-scale) as a numeric CSS environment value on Servo devices, defaulting to 1. Embedders can update the preference through Device::set_preferred_text_scale; CSS that does not reference the variable keeps its original size. Gecko is unchanged.

This is rebased directly onto the Stylo branch currently consumed by Moli, lexmount/stylo:moli at 3c8bbb450a52f9a4dcb2de51061216a1d2727def. The embedder remains responsible for invalidating computed styles when the preference changes.

The change includes a standalone Stylo device/environment test for default and updated values, unknown names, and invalid input, plus a public-CDP Moli test covering computed font size, unaffected fixed font sizes, and preservation across media preference changes. The rebased head was formatted; Moli CI provides the integration build and test result.

Dependency provenance: Moli PR #640 resolves all seven Stylo-family Git dependencies from lexmount/stylo:moli@3c8bbb450a52f9a4dcb2de51061216a1d2727def. This PR targets that exact branch rather than Stylo main, so it retains Moli's existing content / counter() support and other branch-specific fixes.

Moli PR #731 calls Device::set_preferred_text_scale from its style environment update. While this PR is open, Moli pins the contributor fork at immutable commit 3d44ed308. After integration, Moli can return to lexmount/stylo:moli at the resulting commit.

Review illustrations and evidence

完整验证报告：逐项根因、Chromium 对照与原始证据

以下为 ImageGen 生成的机制示意图；执行结果与原始证据见报告。

文字缩放：环境值、样式失效与重新计算

Moli 将缩放偏好传入 Stylo Device 并触发样式重算；Stylo 提供 env(preferred-text-scale)。只有引用该环境值的 CSS 参与缩放，普通固定字号保持原值。

图中 Moli 的环境更新与样式失效由 Moli #731 实现；本 PR 仅负责 Stylo Device 的环境值及默认值。