Crate split phase 2: extract stylo_common (parser context, shared lock, error reporting, atoms, gecko bindings)

[nicoburns]

Summary

Second layer of the crate split (stacked on #6). Adds a new stylo_common crate (lib name style_common) sitting between stylo_traits and style, holding the implementation-support code that can't live in stylo_traits because it depends on gecko bindings or engine support types:

stylo_common
├── parser.rs          (ParserContext, Parse trait — was style/parser.rs)
├── error_reporting.rs
├── shared_lock.rs
├── thread_state.rs
├── use_counters.rs
├── url_extra_data.rs  (UrlExtraData, extracted from style/stylesheets/mod.rs)
├── namespaces.rs      (Namespaces, extracted from stylesheet.rs)
├── attr_taint.rs      (AttrTaint/AttrTaintedRange, extracted from custom_properties.rs)
├── atom_types.rs      (Atom/Namespace/Prefix/LocalName aliases, AtomIdent/AtomString/GenericAtomIdent, extracted from values/mod.rs and lib.rs)
├── gecko_bindings/    (moved wholesale, incl. build_gecko.rs; gecko-only)
└── gecko_string_cache/

The style crate re-exports everything at the old paths (style::parser, style::error_reporting, style::shared_lock, style::Atom, ...), so the public API is unchanged.

Decoupling changes needed to make the layer acyclic:

• error_reporting no longer references the engine's concrete SelectorImpl: the selector-warning visitors are now generic over selectors::parser::SelectorImpl, and ContextualParseError::UnsupportedPropertyDeclaration stores precomputed Vec<SelectorWarningKind> (via new SelectorWarningKind::from_selector_lists) instead of &[SelectorList<SelectorImpl>].
• parser.rs uses selectors::matching::QuirksMode directly instead of the crate::context re-export.
• Since Parse is now a foreign trait to style, impl Parse for InitialValue (= Arc<SpecifiedValue>) hit the orphan rule; replaced with a generic impl<T: Parse> Parse for servo_arc::Arc<T> in stylo_common plus impl Parse for SpecifiedValue in style.
• Stripped ToComputedValue/ToResolvedValue/SpecifiedValueInfo derives from the moved atom types (their traits live above); replaced with trivial_to_computed_value!/trivial_to_resolved_value! in style and manual SpecifiedValueInfo impls.

Verified with cargo check --workspace and cargo test --workspace --exclude stylo (the stylo test target fails on main for a preexisting pseudo-element issue). Gecko-feature code was moved cfg-intact but can't be compile-verified outside mozilla-central.

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

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