feat(style): expose resolved content provenance

[lanyue-llk]

Why

Moli #919 needs style facts that only Stylo can determine after the real cascade and keyframe selection:

• whether the winning visibility value is established on the element or inherited after variable substitution and cascade rollback;
• whether a selected keyframes animation has no opacity effect, uses the underlying effect-stack value, explicitly replaces final opacity, or cannot be resolved.

Rewalking rule nodes or parsing stylesheet text in the consumer loses variable, layer, tree-scope, duplicate-offset, and effect-stack semantics.

Changes

• record HAS_OWN_VISIBILITY after substitution and rollback select the winning declaration;
• expose structured final opacity contribution as NoEffect, Underlying, Replace(value), or Unknown;
• reuse runtime keyframe grouping, variable substitution, and computed-value conversion.

Both APIs are read-only. They add no parser, retained per-node cache, site rule, or case identifier.

Consumer

• lexmount/moli#919 consumes both facts for reader-visible Markdown extraction.

Validation

• cargo +1.96.1 check -p stylo --features servo passed for the implementation before the split;
• the three affected style files are byte-identical to the previously validated combined head;
• Chromium finish() independently confirms reveal then neutral endpoint 1, neutral endpoint then reveal 1, neutral endpoint alone 0, and reveal then a no-fill effect 1;
• Moli integration coverage includes visibility variable outcomes, layer and source priority, duplicate final offsets, document/shadow scope isolation, animation slots, neutral endpoints, effect order, no-forward-fill effects, and paused/infinite boundaries.

This PR is based directly on lexmount/stylo:moli and contains only the resolved content provenance required by Moli #919.

[lanyue-llk]

已将本 PR 的两项修改完整归并到 #1，并保留独立提交与原有验证记录。后续评审和 Moli 依赖统一以 #1 为入口，关闭此重复 PR。

[lanyue-llk]

更正此前“已归并到 #1、#2 重复”的说明：复核后确认两项 PR 解决的是不同问题。#1 是 preferred text scale 环境支持；本 PR #2 提供 Moli #919 实际需要的 resolved visibility / final opacity content provenance，不能由 #1 替代。

本 PR 已同步 Stylo moli 当前提交，最新 head 为 1032a2947ec2f7cee1027d07084203be87e17643，Linux debug/release、macOS debug、Windows debug 与汇总门禁均通过。Moli #919 已精确锁定该提交。此前评论作废，以当前 PR body 与本条更正为准。