fix(joinir): Phase 241-EX - Remove hardcoded 'sum' check from Pattern3

Remove legacy hardcoded 'sum' carrier validation that was blocking
array_filter patterns with different accumulator names (e.g., 'out').

Before: Pattern3 required carrier named 'sum' to exist
After: Pattern3 uses carrier_info generically (any carrier name works)

Test results:
- phase49_joinir_array_filter_smoke: PASS ✅
- phase49_joinir_array_filter_fallback: PASS ✅
- phase49_joinir_array_filter_ab_comparison: PASS ✅
- Full suite: 909/909 PASS, 0 FAIL

Also: Archive old roadmap documentation (67k lines moved to docs/archive/)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs

@@ -252,14 +252,6 @@ impl MirBuilder {
     ) -> Result<Option<ValueId>, String> {
         use crate::mir::join_ir::lowering::loop_with_if_phi_minimal::lower_loop_with_if_phi_pattern;
 
-        // Phase 195: Validate that required carrier 'sum' exists (legacy mode requirement)
-        // This maintains backward compatibility with single-carrier (sum only) and multi-carrier (sum+count) tests
-        let _sum_exists = ctx.carrier_info.carriers.iter()
-            .any(|c| c.name == "sum");
-        if !_sum_exists {
-            return Err("[cf_loop/pattern3] Accumulator variable 'sum' not found in variable_map".to_string());
-        }
-
         // Phase 195: Use unified trace
         trace::trace().varmap("pattern3_start", &self.variable_map);