Crate split phase 1: stylo_build codegen crate + property IDs and small types into stylo_traits

[nicoburns]

Summary

Phase 1 of splitting the stylo crate into smaller crates for compile-time (pipelining) wins. This PR moves the codegen pipeline and the "small types everything names" down below the main crate, keeping the style:: API unchanged via re-exports.

• New stylo_build crate: the mako/python property-codegen pipeline (build.py, data.py, templates, property TOMLs, vendored python) moves out of style/ into a shared [build-dependencies] crate exposing generate_properties(engine) and a new generate_property_ids(engine) target. Single source of truth for property data; multiple crates can now run codegen into their own OUT_DIR.
• Property ID enums generated into stylo_traits: a new property_ids.mako.rs template renders LonghandId, ShorthandId, NonCustomPropertyId, AliasId, LogicalGroupId, PrioritaryPropertyId, CountedUnknownProperty, property_counts, and the ID-set machinery (LonghandIdSet etc.) into style_traits::property_ids.
• Higher-layer ID methods become extension traits in the generated properties code (since inherent impls must live with the type):

  // generated in style::properties
  trait NonCustomPropertyIdExt { fn allowed_in(&ParserContext)...; }
  trait LonghandIdExt { fn parse_value(...) -> PropertyDeclaration; fn to_physical(WritingMode)...; }
  trait ShorthandIdExt { fn parse_into(...); fn longhands_to_css(...); }
  trait ShorthandIdSerializationExt { fn get_shorthand_appendable_value(...); }

  ToComputedValue/ToResolvedValue for the ID enums are now trivial_to_computed_value!/trivial_to_resolved_value! in the main crate instead of derives.
• Small neutral types move to stylo_traits: Origin/OriginSet/PerOrigin (style_traits::origin), CssRuleType/CssRuleTypes/CorsMode (style_traits::rule_types). Old paths re-exported.
• Gecko's property-enabled pref check (which reads a gecko_bindings static) is behind a runtime-installed function pointer (set_gecko_property_enabled_fn) so stylo_traits doesn't depend on the bindings.

No behavior change intended; style::properties::LonghandId etc. remain available at their old paths.

Later phases (stacked on this PR) will extract stylo_common, stylo_values, and stylo_properties.

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

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