feat(css): expose preferred text scale in the Servo environment

[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 extends the exact revision currently pinned by Moli (wpt/property-rule-escaped-names); it does not pull in unrelated Stylo changes. The embedder remains responsible for invalidating computed styles when the preference changes.

Validated with rustfmt, a standalone Stylo device/environment test for default and updated values, unknown names, and invalid input, plus a public-CDP Moli test that checks 1.5 → 2 → reset, computed font size and width, unaffected fixed font sizes, and preservation across media preference changes. Chromium 151 was used as the behavioral reference.

Dependency provenance: Moli PR #640 pins all seven Stylo-family Git dependencies to ldm0/stylo@671d13d31b3b00e7b1e0088ba2b36a4e35c0858c; that is the head of this PR’s target branch wpt/property-rule-escaped-names. The repository is an active fork of servo/stylo, not an unused substitute. Default branch main is not the revision Moli currently consumes.

Moli PR #731 calls Device::set_preferred_text_scale from its style environment update. While this PR is open, Moli pins the contributor fork lanyue-llk/stylo at immutable commit 402a73dba5624bbfb0a16639f03bd48db63af1b0. After this change is integrated into the consumed branch, the dependency can return to ldm0/stylo using the actual resulting commit and updated lockfile; this PR does not switch Moli to Stylo main.

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。

[lanyue-llk]

已核对 PR 目标、Stylo 现有 env() 路径、Servo Device setter 约定及 CSSWG 的 preferred-text-scale 定义。改动范围小且分层合理；下面两处分别涉及公开 API 的输入契约和本仓库的回归保护。

[lanyue-llk]

该 PR 的目标分支不是 Moli 实际消费的 Stylo 基线。文字缩放提交已真正 rebase 到 lexmount/stylo:moli@3c8bbb450a52f9a4dcb2de51061216a1d2727def，由 lexmount#1 取代；Moli #731 也已固定到新的不可变提交。