Crate split phase 3c: untangle StyleBuilder's Stylist and Device upward references

[nicoburns]

Summary

Removes three of the upward references that block extracting the generated "types half" (style structs / ComputedValues / StyleBuilder, see #10) into a future stylo_values crate:

1. StyleBuilder.stylist no longer names Stylist. All the value-computation layer ever does with it is look up registered custom properties, so:

// values::computed (low layer)
pub trait CustomPropertyRegistry {
    fn get_custom_property_registration(&self, name: &Atom) -> &Descriptors;
    fn get_custom_property_initial_values(&self) -> &ComputedCustomProperties;
}

// StyleBuilder (generated types half)
- pub stylist: Option<&'a Stylist>,
+ pub stylist: Option<&'a dyn computed::CustomPropertyRegistry>,

Stylist implements the trait; the custom-property substitution chain (custom_properties::substitute*, UnparsedValue::substitute_variables, container-query custom-property comparisons) now takes &dyn CustomPropertyRegistry, resolving env() through the computed context's device instead of stylist.device(). The two cascade sites that need the concrete Stylist (cascade_rules for visited styles, variable substitution) use Cascade's own stylist field instead of going through the builder. Context::new_for_initial_at_property_value takes (device, registry, quirks_mode) instead of &Stylist.

2. Device no longer names GeckoElement (the DOM-layer wrapper): Device::calc_line_height and ResolvedElementInfo.element take the raw gecko_bindings::structs::Element reference (which lives at the bottom of the graph); the wrapper was only ever unwrapped to that pointer for FFI. style/device/ now has zero references to upper layers (stylist, rule tree, stylesheets, DOM wrappers), so it can move down with the types half.

3. The gecko system_font module is generated in the types half (properties::system_font, extracted from gecko.mako.rs into system_font.mako.rs), with a compat re-export at properties::gecko::system_font. This makes Context.cached_system_font's type live below the property machinery.

Remaining knots for extraction (next phase): ComputedValues.rules: Option<StrongRuleNode> (rule tree depends on PropertyDeclarationBlock) and PseudoElement placement.

Validated with cargo check --workspace, cargo test --workspace --exclude stylo (the stylo test target has a pre-existing failure on main), and gecko template rendering.

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

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