feat: Spock-style implicit assertions for Then/Expect blocks

[JPDuchesne]

Summary

• Spock-style implicit assertions: every non-assignment statement in Then/Expect blocks is now an assertion — no assertion API needed, matching Spock's core model.
• Binary operator assertions: ==, !=, =~, !~, >, <, >=, <= map to specialized Minitest assertions (assert_equal, refute_equal, assert_match, refute_match, assert_operator) with clear error messages.
• General statement assertions: bare boolean expressions (e.g. obj.valid?) are wrapped in assert_equal(true, expr) with the original source text in the error message. Negation (!expr) is detected and maps to assert_equal(false, inner_expr).
• New pipeline: StatementParser classifies statements at parse time; StatementToAssertionTransformation handles Minitest mapping. Replaces the old ComparisonToAssertionTransformation.
• Documentation: README updated with assertion tables, examples, and expanded Then/Expect block docs. CHANGELOG backfilled for all prior versions (1.0.0 through 2.3.1). Release instructions updated to remind about CHANGELOG maintenance.

Test plan

• bundle exec rake test passes (unit + integration tests)
• New StatementParserTest covers assignment passthrough, binary operator classification, and general statement wrapping with source text
• New StatementToAssertionTransformationTest covers all 8 binary operators, general statements, and negation
• Updated ThenBlockTest and ExpectBlockTest verify new node types
• Integration tests in ExampleRSpockTest exercise =~, >, <, >=, <=, bare predicates, negation, and variable assignments in Expect/Then blocks
• CHANGELOG renders correctly on GitHub with comparison links

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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