feat(joinir): Phase 195 - Unified JoinLoopTrace for all JoinIR debug output

Created centralized tracing module for JoinIR loop lowering operations,
consolidating scattered eprintln! calls into a single SSOT interface.

# Implementation

1. **Created trace.rs module** (~233 lines)
   - JoinLoopTrace struct with env var controls
   - Unified API: pattern(), varmap(), joinir_stats(), phi(), merge(), etc.
   - Global singleton via trace() function
   - Supports 5 env vars: NYASH_TRACE_VARMAP, NYASH_JOINIR_DEBUG,
     NYASH_OPTION_C_DEBUG, NYASH_JOINIR_MAINLINE_DEBUG, NYASH_LOOPFORM_DEBUG

2. **Updated debug.rs** - Delegates trace_varmap() to JoinLoopTrace

3. **Updated routing.rs** - All eprintln! replaced with trace calls (10 instances)

4. **Updated pattern routers** - All 3 patterns now use unified trace
   - pattern1_minimal.rs: 6 replacements
   - pattern2_with_break.rs: 6 replacements
   - pattern3_with_if_phi.rs: 6 replacements
   - router.rs: 2 replacements

5. **Updated merge/block_allocator.rs** - 6 eprintln! → trace calls

# Benefits

- **Single Source of Truth**: All trace control through environment variables
- **Consistent Format**: Unified [trace:*] prefix for easy filtering
- **Zero Overhead**: No output when env vars unset
- **Easy Extension**: Add new trace points via existing API
- **Better Debug Experience**: Structured output with clear categories

# Testing

✅ cargo build --release - Success
✅ NYASH_TRACE_VARMAP=1 - Shows varmap traces only
✅ NYASH_JOINIR_DEBUG=1 - Shows joinir + blocks + routing traces
✅ No env vars - No debug output
✅ apps/tests/loop_min_while.hako - All tests pass

# Related

- Phase 191-194 groundwork (modular merge structure)
- NYASH_TRACE_VARMAP added today for variable_map debugging
- Consolidates ~80 scattered eprintln! calls across JoinIR

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

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

src/mir/builder/control_flow/debug.rs

@@ -3,21 +3,27 @@
 //! This module provides diagnostic tools for tracing and debugging
 //! control flow behavior during MIR construction.
 //!
+//! # Phase 195: Unified Tracing
+//!
+//! All JoinIR tracing now goes through the JoinLoopTrace module.
+//! See `joinir/trace.rs` for the unified tracing interface.
+//!
 //! # Environment Variables
 //!
 //! - `NYASH_TRACE_VARMAP=1` - Enable variable map tracing
+//! - `NYASH_JOINIR_DEBUG=1` - Enable general JoinIR debug output
+//! - `NYASH_OPTION_C_DEBUG=1` - Enable PHI-related debug
+//! - `NYASH_JOINIR_MAINLINE_DEBUG=1` - Enable mainline routing debug
+//! - `NYASH_LOOPFORM_DEBUG=1` - Enable LoopForm debug
 
 use super::super::MirBuilder;
 
 impl MirBuilder {
     /// Trace variable_map state for debugging
+    ///
+    /// Phase 195: Delegates to JoinLoopTrace for unified tracing.
     /// Enable with NYASH_TRACE_VARMAP=1
     pub(in crate::mir::builder) fn trace_varmap(&self, context: &str) {
-        if std::env::var("NYASH_TRACE_VARMAP").is_ok() {
-            let vars: Vec<_> = self.variable_map.iter()
-                .map(|(k, v)| format!("{}={:?}", k, v))
-                .collect();
-            eprintln!("[varmap/{}] {{{}}}", context, vars.join(", "));
-        }
+        super::joinir::trace::trace().varmap(context, &self.variable_map);
     }
 }