feat(joinir): Phase 183 LoopBodyLocal role separation + test fixes

Phase 183 Implementation:
- Added is_var_used_in_condition() helper for AST variable detection
- Implemented LoopBodyLocal filtering in TrimLoopLowerer
- Created 4 test files for P1/P2 patterns
- Added 5 unit tests for variable detection

Test Fixes:
- Fixed test_is_outer_scope_variable_pinned (BasicBlockId import)
- Fixed test_pattern2_accepts_loop_param_only (literal node usage)

Refactoring:
- Unified pattern detection documentation
- Consolidated CarrierInfo initialization
- Documented LoopScopeShape construction paths

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -15,6 +15,7 @@ default = ["cli", "plugins"]
 # Legacy features removed - archive cleaned up
 e2e = []
 cli = []
+legacy-tests = [] # Legacy/phi-off test suites (pre-JoinIR). Disabled by default.
 plugins-only = []
 builtin-core = []
 builtin-filebox = []  # Enable built-in FileBox provider (SSOT)