[Content Addressable Gems] Replace hash/array manipulation with named classes

Context

In (ruby/rubygems#9773), there was a suggestion giving some of the hashes and arrays names — i.e. making concrete classes with names.

The branch has a lot of pre exisiting hash and array manipulation: positional rows indexed by magic constants, composite array keys, and parallel nested hashes threaded through long method chains. Two ideas to start:

1. Gem::CompactIndexClient::InfoRow

Compact index info rows are 5-element arrays indexed via INFO_NAME = 0 … INFO_REQS = 4. Wrap them in a Struct with named accessors (#version, #suffix, #ruby_requirement, #platform_requirement, #created_at) and convert at the consumption boundary via InfoRow.wrap, so older Bundler-shipped copies of the client (which return plain arrays behind the load guard) keep working. Parser hot path is unchanged; INFO_* constants stay for compatibility.

2. Gem::QueryUtils::VersionPlatforms

gem list/search/info build two parallel auto-vivifying hashes (platforms and platform_ruby_abis) and pass them together through six methods. Replace both with one presenter exposing #versions, #platforms_for(version), #ruby_abis_for(version, platform), and #ruby_abi_metadata?, dropping the paired parameters and the repeated values.any? { .values.any?(&:any?) } crawls.

There are more candidates (e.g. the throwaway row arrays and composite keys in Gem::Source#decode_content_addressable_tuples, the stub-target: hash in StubSpecification), which can follow the same pattern later.

Start by tracing Gem::CompactIndexClient::InfoRow and Gem::QueryUtils::VersionPlatforms through their callers, including the six methods that pass the parallel hashes. Introduce the named interfaces described in the issue while preserving INFO_* compatibility and support for older Bundler-shipped clients; done means the paired parameters and repeated nested-hash traversal are removed.