Claim slice 4a: Ruby include/extend/prepend as captures on the shared base-relations path #400

I'd like to claim slice 4a from Adding a language: common stays common, following the maintainer's offer on #339 — the extraction PR (#396) has since landed.

Problem

Ruby's include/extend/prepend reach the base-relations path through a Ruby-only function, captureRubyMixinBases (src/ingest_relations.h), called from a lang == Lang::Ruby arm of the tags pass. The shared path (captureBases + isBaseTypeNodeIn) already handles the base clause of every other language — and Ruby's own < superclass — from the class node's clause children. The mixin directives are the one Ruby shape that bypasses it.

CONTRIBUTING.md row 4 lists this as: "4a Ruby include/extend/prepend as captures read by the shared path, retiring captureRubyMixinBases: S–M, Ruby."

Scope

• Capture the mixin directive in queries/ruby/tags.scm under a shared, language-neutral capture name, structurally: the tags pass does not evaluate predicates (#eq?/#match? are silently ignored — pinned by test/matchgrammarcheck.sh; generalising the evaluator is slice 5a, not claimed here). So the pattern carries the shape, and C++ owns only the guards the query cannot express — the same division the shared import vocabulary uses for require (Shared import-capture vocabulary instead of ~21 per-language import extractors #358).
• Route that capture through the shared base-relations path, so each mixin constant becomes an inherit ref exactly as today, and delete captureRubyMixinBases and its call site rather than leaving them beside the shared path.
• No new Lang:: branch in shared code; no language-named capture kind.
• Preserve — or pin as known gaps — the floors the current code states: a begin- or modifier-if-guarded directive, the do-block bodies of included/class_methods/Struct.new/Class.new/Module.new, and class << Registry (another object's singleton).

Acceptance

• Red-first: a new arm that is red on main before the change; test/rubyinheritcheck.sh and test/chacheck.sh green.
• Byte-identical output on every language the shared path already handles — the map, --deps --limit=100000, --skipped, and --callers/--callees/--impact/--uses on symbols the slice touches — over the fixtures of every language touched plus at least one real repository per language. Empty diff, or each difference named as intended.
• ripwire src --clones no longer lists the group the slice folded; ripwire . --quality-delta=origin/main..HEAD gates 0.
• test/qschemetripcheck.sh green, with kParserVer and its mirror bumped above main's (maintainers set the final number at merge).
• A known-gap arm for anything the slice does not fix.

Relationship to other slices

Not 4b (base-clause node kinds moved into tags.scm) and not 4c (structural conformance). It does not depend on slice 5a: the design assumes predicates do not run.