Crate split phase 3b: split generated properties into a types half and a machinery half

[nicoburns]

Summary

Implements the "option 3" cut of the generated property code: properties.mako.rs now renders twice into two files that are both include!d into the same crate::properties::generated module, so every existing path keeps working while the generated code is physically partitioned along the future stylo_values / stylo_properties crate boundary.

• properties_structs.rs (types half, ~56k lines servo): longhand value types (longhands::<prop>::{SpecifiedValue, computed_value, parse, get_initial_value, ...}), style_structs, ComputedValues/ComputedValuesInner, StyleStructRef, StyleBuilder.
• properties.rs (machinery half, ~63k lines servo): PropertyDeclaration, shorthands, LonghandId/ShorthandId/PropertyId ext impls + logical mapping, CASCADE_PROPERTY/parse tables, animated_properties, gecko module, descriptors, size tests.

Key mechanical changes:

• build.py renders the template with half="types" and half="machinery"; sections are wrapped in % if half == ... guards.
• The per-longhand cascade_property / parse_declared functions move out of longhands::<prop> into a new generated longhands_machinery::<prop> module (they reference PropertyDeclaration); the generated dispatch tables now point there. Nothing handwritten referenced them directly.
• ComputedValues methods that need PropertyDeclaration/resolved-serialization (computed_or_resolved_value, property_value_to_typed_value_list, computed_or_resolved_declaration, transition_properties) move to a machinery-side impl ComputedValues block.
• Vector longhands' SpecifiedValue::compute_iter becomes pub so the machinery half can call it.

Remaining upward references in the types half (to be untangled before the actual crate extraction, next PR): StyleBuilder.stylist: Option<&Stylist>, ComputedValues.rules: Option<StrongRuleNode>, pseudo: Option<PseudoElement>, custom_properties, and the gecko style structs (still generated in the machinery half's gecko module and re-exported).

Verified: cargo check --workspace clean (0 warnings), cargo test --workspace --exclude stylo passes, gecko-engine template renders successfully.

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

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