fix(frontend): allow typed object nil equality

[SuperIceCN]

Summary

This PR fixes frontend semantic resolution for typed object and null equality so ordinary nullable-object guards compile without sentinel workarounds. It aligns frontend typing and compile checks with the existing backend nil-comparison path.

What changed

• Added a narrow frontend semantic rule so == and != between typed object values and Nil resolve to bool instead of falling through to builtin operator metadata failure.
• Updated frontend compile/lowering flow coverage to keep typed object null guards lowering-ready.
• Added frontend, lowering, backend, and test-suite coverage for object-nil equality and for rejecting unsupported ordering operators.
• Restored benchmark/test scripts to use direct nullable-object checks instead of sentinel-style workarounds.
• Folded the final semantic notes back into the frontend unary/binary expression implementation document.

Why

• Issue frontend: typed object null equality is rejected before nil comparison lowering #44 exposed a frontend/backend contract mismatch: backend codegen already supports object-nil comparison, but frontend semantic typing rejected the same operand pair before lowering.
• That rejection blocked normal source shapes such as current != null in typed linked structures and forced benchmark code to avoid ordinary nullable-object guards.
• Keeping the rule narrow to equality operators preserves existing operator constraints while making typed object null checks compile as expected.

Affected packages/files

• gd.script.gdcc.frontend.sema.analyzer
• gd.script.gdcc.frontend.sema.analyzer.support
• gd.script.gdcc.frontend.lowering.cfg.item
• gd.script.gdcc.backend.c.gen
• src/test/test_suite/unit_test/script/smoke/object_nil_equality.gd
• doc/module_impl/frontend/frontend_unary_binary_expr_semantic_implementation.md

Validation

• ./gradlew test --tests FrontendCompileCheckAnalyzerTest --tests FrontendExpressionSemanticSupportTest --tests FrontendLoweringBodyInsnPassTest --tests COperatorInsnGenTest --tests GdScriptUnitTestCompileRunnerTest --no-daemon --info --console=plain
• ./gradlew classes --no-daemon --info --console=plain

Result: BUILD SUCCESSFUL

Risks / Notes

• The semantic special case remains intentionally narrow: only == and != for object-Nil and Nil-Nil pairs are accepted.
• Ordering operators for object-nil pairs still fail and remain covered by tests.
• The PR also includes the benchmark/test fixture cleanup needed to exercise the real nullable-object source shape.

Key behaviors covered (Optional)

• typed_object == null and typed_object != null resolve to bool.
• null == typed_object and null != typed_object resolve to bool.
• null == null and null != null resolve to bool.
• typed_object < null remains rejected.
• Typed object while current != null guards compile through frontend/lowering and reach backend nil-comparison handling.

Diff stats (Optional)

• 15 files changed, 403 insertions, 39 deletions.

Breaking changes (Optional)

• None

Related docs (Optional)

• doc/module_impl/frontend/frontend_unary_binary_expr_semantic_implementation.md

Closes #44