Crate split phase 3a: funnel all values -> engine accesses through computed::Context methods

[nicoburns]

Summary

First slice of the stylo_values extraction (stacked on #7): make values::computed::Context the single chokepoint between value-computation code and the engine layers above it, so the future ComputationContext trait boundary is exactly the set of inherent methods on Context.

• style/values/** no longer pokes into context.builder, context.style() or context.device() directly. All such accesses (~40 sites) go through new narrow Context methods:

  impl<'a> Context<'a> {
      fn effective_zoom(&self) -> Zoom;
      fn writing_mode(&self) -> WritingMode;
      fn inherited_writing_mode(&self) -> WritingMode;
      fn parent_writing_mode_property(&self) -> WritingModeProperty;
      fn parent_text_align(&self) -> TextAlign;
      fn parent_color(&self) -> AbsoluteColor;
      fn parent_font_weight(&self) -> FontWeight;
      fn parent_font_size_keyword_info(&self) -> KeywordInfo;
      fn parent_math_depth(&self) -> i8;
      fn parent_math_style_is_compact(&self) -> bool;
      fn font_size(&self, base: FontBaseSize) -> FontSize;
      fn line_height(&self, base: LineHeightBase) -> NonNegativeLength;
      fn default_line_height(&self) -> NonNegativeLength;
      fn base_size_for_font_size_keyword(&self) -> Length;
      fn add_flags(&self, flags: ComputedValueFlags);
      fn note_writing_mode_dependency(&self);
      fn note_color_scheme_dependency(&self);
      fn color_scheme(&self) -> ColorSchemeFlags;
      fn is_dark_color_scheme(&self) -> bool;
      fn system_color(&self, c: SystemColor) -> AbsoluteColor;   // servo
      fn system_nscolor(&self, c: SystemColor) -> u32;           // gecko
      fn body_text_color(&self) -> AbsoluteColor;
      fn root_font_size/root_line_height/root_font_metrics_{ex,ch,cap,ic}(&self) -> Length;
      fn app_units_per_device_pixel(&self) -> i32;
      fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel>;
      fn is_supported_mime_type(&self, mime: &str) -> bool;
      fn chrome_rules_enabled_for_document(&self) -> bool;
  }

  All method bodies are the exact expressions previously inlined at the call sites (no behavior change); e.g. FontBaseSize::resolve now just delegates to context.font_size(*self).

• Move the element-context support types down so Context's fields stop pointing at the dom/context layers:

  • ElementContext, DummyElementContext, AttributeTracker, AttributeReferences, TreeCountingCaches, TreeCountingResult → stylo_common::element_context (re-exported from their old style::dom / style::context / values::computed paths).
  • The old blanket impl<T: TElement> ElementContext for T can't move (orphan rule) and is replaced by a TElementContext<E>(pub E) adapter in style::dom, used at the single coercion site in properties::cascade.
  • PrecomputedHasher/PrecomputedHashMap/PrecomputedHashSet → style_traits::precomputed_hash (re-exported from style::selector_map).

• Redirect values/ imports of already-moved types to their real homes: LonghandId/LogicalGroupId etc. from style_traits::property_ids, CorsMode from style_traits::rule_types, QuirksMode from selectors::matching, and SpecifiedWritingMode from logical_geometry::WritingModeProperty instead of the generated longhands::writing_mode alias.

After this, the only remaining values -> upper layer couplings are inside computed::Context/animated::Context/resolved::Context themselves (StyleBuilder, ComputedValues, Device, Stylist, container-query support types) plus a handful of property-ID parse entry points — i.e. the surface that the actual crate boundary will have to abstract.

Verified with cargo check --workspace and cargo test --workspace --exclude stylo (stylo test target fails on main for a preexisting pseudo-element issue).

Link to Devin session: https://app.devin.ai/sessions/5d2d6be20e724623bdc1bcf7e1687af1
Requested by: @nicoburns

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring