Fix Hash#fetch generic leak from unmatched RBS interface overloads

[apiology]

This PR was written by Claude Code on behalf of @apiology.

Fixes castwide#1227

Summary

Pin::Parameter#compatible_arg? rejected every argument against an RBS-interface-typed parameter (e.g. Hash#fetch's _Key) unless the argument's type was nominally included via CoreFills::INCLUDES. Since Hash#fetch's plain (no default, no block) overload takes a _Key-typed parameter, no overload of h.fetch(k) ever matched during argument-type inference, and Pin::Method#return_type fell back to unioning every overload's return type together — leaking the unbound generic X from the default-value and block overloads into the inferred type.

# @param h [Hash{String => Integer}]
# @return [Integer]
def fetch_it(h)
  h.fetch('a')
end

was reported as inferring ::Integer, generic<X> against the declared ::Integer return.

Fix

Add :allow_unmatched_interface to the rules passed to conforms_to? in compatible_arg?, matching the leniency TypeChecker already applies by default everywhere except its :alpha level (Rules#require_interfaces_resolved?). This is scoped to overload argument matching only — compatible_arg? has a single call site (Source::Chain::Call#inferred_pins).

Test plan

• Added specs in spec/type_checker/levels/strong_spec.rb covering Hash#fetch (fixed) and Hash#[] (control, unaffected)
• bundle exec rspec — 1626 examples, 0 failures, 60 pending (no new failures)
• bundle exec rubocop on changed files — no offenses