Problem

The AI optimizer sometimes generates "optimizations" that add new fields or helper methods to a Java class without changing the target method at all. Because benchmark noise produces small timing variations, these fake optimizations pass the speedup critic and create PRs with no real improvement.

Example: 4 commons-lang PRs each added private static final Supplier<String> NULL_SUPPLIER = Suppliers.nul(); but the target methods (getJavaAwtHeadless, getJavaIoTmpdir, etc.) were never modified to use it — yet reported 7-151% speedups.

Root Cause

replace_function() in replacement.py accepts any optimization that changes the file, even if the target method body is identical to the original. The dedup check compares the entire candidate (function + helpers/fields), so adding a new field makes it "different" from the original, bypassing the identity check.

Fix

Added _has_unused_additions() in replacement.py that:

1. Compares the target method body before and after optimization (whitespace-normalized)
2. If unchanged AND new fields/helpers exist, checks if any addition identifiers are referenced in the target method body
3. Rejects the candidate if additions are unreferenced, returning the original source unchanged

This causes replace_function_definitions_for_language() to return False (no update), which skips the candidate.

Validation

• Unit tests: 37/37 pass (5 new + 32 existing, zero regressions)
• E2E: Fibonacci optimization ran without false rejections — no "unreferenced" warnings in logs
• Session: /home/ubuntu/e2e-sessions/2026-04-01_15-45_cf1081-unused-additions/

Test Coverage

New TestUnusedAdditionsRejection class with 5 tests:

• test_unchanged_method_with_unused_field_rejected — unchanged method + unused field → rejected
• test_unchanged_method_with_unused_helper_rejected — unchanged method + unused helper → rejected
• test_changed_method_with_used_field_accepted — changed method + used field → accepted
• test_changed_method_without_additions_accepted — normal optimization → accepted
• test_unchanged_method_with_used_helper_accepted — method uses new helper → accepted

Closes CF-1081