Internalize versioning state behind Grape::Util::InheritableSetting accessors

[ericproulx]

Second batch of the namespace_inheritable cleanup (after #2805; #2795–#2804 covered the stackable side): the versioning state written by the version, prefix and cascade DSL methods is now recorded and read through intention-revealing accessors on Grape::Util::InheritableSetting.

New accessors on InheritableSetting

Accessor	Replaces
version / version=	namespace_inheritable[:version]
version_options / version_options=	namespace_inheritable[:version_options]
root_prefix / root_prefix=	namespace_inheritable[:root_prefix]
cascade / cascade= / cascade_defined?	namespace_inheritable[:cascade] / .key?(:cascade)

Notes:

• cascade is the one subtle key: an explicitly assigned nil is meaningful and distinct from never-set (InheritableValues#[] is key-presence based via fetch), so the accessor family includes cascade_defined?. Both presence checks — DSL::Routing#cascade's getter and Grape::API::Instance#cascade?'s fallback chain (explicit cascade → version_options.cascade → true) — now read through it, and the semantics are documented on the accessor instead of implied at two call sites.
• cascade lives in this PR rather than a flags PR because Instance#cascade? also reads version_options — keeping the whole fallback chain in one conversion avoids two standalone PRs editing the same method.
• Converted call sites: DSL::Routing (version, prefix, cascade), Endpoint (to_routes, build_stack versioner setup — the reader absorbs nothing here since .flatten / .present? are caller concerns), Grape::API::Instance#cascade?, and the DSL::VersionOptions doc comment.
• These are nearest-wins scalars, so writers use plain = (see Internalize format and error-response defaults behind Grape::Util::InheritableSetting accessors #2805 for the naming rationale).
• Storage under the raw keys is unchanged; they should now be considered internal. Specs assert through the accessors.

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