refactor: introduce formal RSpock AST with parse-transform pipeline

[JPDuchesne]

Summary

• Restructure blocks into RSpock::AST::Parser namespace — Move block accumulators (Block, GivenBlock, WhenBlock, etc.) under Parser, convert InteractionParser from module to class, eliminate StartBlock/EndBlock in favor of declarative can_start?/can_end? on each block, and rename source_map to block_registry.

• Introduce formal RSpock AST node hierarchy — Create RSpock::AST::Node subclasses (TestNode, DefNode, BodyNode, WhereNode, InteractionNode, etc.) backed by a type registry and NodeBuilder module, extract TestMethodParser to own the parse phase, and make TestMethodTransformation stateless (operating on TestNode → Ruby AST). TestNode uses positional children [DefNode, BodyNode, WhereNode?] for clean structural separation.

• Add interaction transformations and block identity verification — Implement InteractionToMochaMockTransformation and InteractionToBlockIdentityAssertionTransformation operating on InteractionNode, add RSpock::Helpers::BlockCapture for runtime block capture, delete legacy InteractionTransformation (which violated the AbstractTransformation contract) and InteractionBlockAssertionTransformation (redundant abstraction), and port all missing test coverage.

• Block forwarding verification in interactions — When testing code that forwards a block to a collaborator (e.g. a method wrapping a UI frame or transaction), there was previously no way to verify that the correct block was actually passed through. This PR introduces &var syntax in interactions to assert block identity:

  test "forwards the block to the collaborator" do
    Given
    my_proc = -> { "work" }
  
    When
    subject.do_work(&my_proc)
  
    Then
    1 * @collaborator.perform(&my_proc)
  end

  This transforms into a Mocha expectation plus a BlockCapture that intercepts the block at runtime, followed by an assert_same to verify the exact proc was forwarded — not just that a block was passed, but that the right one was. Inline blocks (do...end / { }) are explicitly rejected with a clear error, since there is no external reference to compare identity against.

Test plan

• Full test suite passes (182 tests, 266 assertions, 0 failures, 0 errors)
• Verify CI passes on all supported Ruby versions

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.94068% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ck/ast/interaction_to_mocha_mock_transformation.rb	96.55%	2 Missing ⚠️
lib/rspock/ast/parser/interaction_parser.rb	96.29%	2 Missing ⚠️
lib/rspock/ast/parser/test_method_parser.rb	98.07%	1 Missing ⚠️

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