refactor(compiler/tests): restructure test layout and relocate test-only helpers to test source set

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Context

The :modules:compiler:core test suite currently has two structural problems that make it harder to navigate and maintain as coverage grows.

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Problem 1 — Test helpers live in src/main

Three files that exist purely to support tests are compiled into the main source set:

• src/main/java/com/orca/compiler/core/tests/CompilerTestHelper.java
• src/main/java/com/orca/compiler/core/tests/SourceBuilder.java
• src/main/java/com/orca/compiler/core/tests/TopLevelSourceBuilder.java

Because they are in src/main, they end up on the production classpath and are included in any build artifact. They should live under src/test/java/…/tests/ instead, where they are only compiled for the test task and never shipped.

Goal: move all three files to src/test/java/com/orca/compiler/core/tests/ and confirm the test suite still passes.

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Problem 2 — All test classes are flat and some are too large

All five test classes sit directly in com.orca.compiler.core with no sub-grouping. Two of them are approaching 1 000 lines:

File	Lines	Sections
TestCodeGen.java	~992	16 sections
TestTypeChecker.java	~910	10 sections
TestParser.java	~390	—
TestLexer.java	~283	—
TestTypeSystem.java	~220	—

Splitting by compiler phase and logical grouping would mirror the structure already used in src/main and make it clear where to add or remove tests.

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Proposed layout

src/test/java/com/orca/compiler/core/
    tests/                            ← helpers moved from src/main
        CompilerTestHelper.java
        SourceBuilder.java
        TopLevelSourceBuilder.java
    lexer/
        LexerTest.java
    parser/
        ParserTest.java
    typesystem/
        TypeSystemTest.java
    semantic/
        SemanticTest.java             ← structural typing, type checks (valid + multi-source)
        SemanticScopeTest.java        ← scope/declarations, definite assignment, return statements
        SemanticDeclTest.java         ← constants, operators, impl blocks, collections
    codegen/
        ExpressionCodeGenTest.java    ← literals, arithmetic (int+float), strings, booleans, comparisons
        ControlFlowCodeGenTest.java   ← variables, if/else, while, for
        FunctionCodeGenTest.java      ← functions, impl methods, built-in functions
        CollectionCodeGenTest.java    ← arrays, collections
        IntegrationCodeGenTest.java   ← any type, compound/integration tests

The split boundaries above are proposals — feel free to adjust groupings during implementation.

TestTypeChecker is renamed to SemanticTest (and friends) because the test class covers the full semantic analysis phase, which lives entirely in the Binder — there is no distinct TypeChecker pass.

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Acceptance criteria

• CompilerTestHelper, SourceBuilder, and TopLevelSourceBuilder are moved to src/test
• Test classes are split into sub-packages that reflect compiler phases
• All existing tests pass without modification to test logic
• No test-only code remains in src/main