test(ezc): comprehensive test coverage

[SchoolyB]

Summary

Adds 50 new tests across two new test suites and fixes a regression in return type parsing.

New: Type Checker Tests (26 tests)

• Scope: define, lookup, nesting, shadowing, undefined
• Type constructors: primitives, arrays, structs, numeric checks
• Expression resolution: literals, arithmetic, comparisons, logical, negation, arrays
• Builtins: len, typeof, to_float
• Struct field and function return type resolution

New: End-to-End Codegen Tests (24 tests)

Compile EZ snippets → run binaries → verify output:

• Hello world, arithmetic, variables, string interpolation
• Control flow: if/or/otherwise, for/range, while, loop/break
• Functions: calls, recursion, multi-return, mutable params, ensure
• Types: structs, enums, arrays, array mutation, for_each, when/is
• Builtins: len, typeof, compound assignment, char, blank identifier

Fix: Return Type Parsing Regression

-> (int, int) was incorrectly parsed as a shared-type named return (treating int as a variable name). Added type name detection to disambiguate plain types from variable names in return type lists.

Test Targets

• make test-unit — 61 tests (lexer + parser + type checker)
• make test-e2e — 24 tests (compile and run)
• make test — all 85 tests + parity

Results

85/85 tests passing