refactor(joinir): Phase 260 P0.1 Step 2 - Extract logging.rs (DEBUG-177 style)

Extract logging utilities into dedicated module (safest first split).
Provides log_if() function + rewriter_log!() macro as alternatives to local log! macro.

Changes:
- NEW: rewriter/logging.rs - log_if() + rewriter_log!() macro
- CHANGED: rewriter/mod.rs - declare logging module

Design:
- log_if(trace, enabled, msg): Function form for explicit control
- rewriter_log!(): Macro form for format!() convenience
- Both wrap JoinLoopTrace::stderr_if() (existing infrastructure)

Status:
- instruction_rewriter.rs still uses local log! macro (unchanged)
- Future: Gradually migrate to logging::log_if() or rewriter_log!()
- cargo check PASS ✅

Next: Extract is_skippable_continuation to helpers.rs (smallest function)

src/mir/builder/control_flow/joinir/merge/rewriter/logging.rs

@@ -0,0 +1,54 @@
+//! Logging utilities for instruction rewriter
+//!
+//! Phase 260 P0.1: Extracted from instruction_rewriter.rs
+//! Provides centralized logging with verbose flag control (DEBUG-177 style).
+
+use crate::mir::builder::control_flow::joinir::trace::JoinLoopTrace;
+
+/// Log message conditionally based on enabled flag
+///
+/// Equivalent to the log! macro in instruction_rewriter.rs:
+/// ```
+/// macro_rules! log {
+///     ($enabled:expr, $($arg:tt)*) => {
+///         trace.stderr_if(&format!($($arg)*), $enabled);
+///     };
+/// }
+/// ```
+///
+/// # Example
+///
+/// ```ignore
+/// let trace = trace::trace();
+/// log_if(&trace, true, "[DEBUG] Block {:?} processed", block_id);
+/// ```
+#[inline]
+pub(super) fn log_if(trace: &JoinLoopTrace, enabled: bool, message: &str) {
+    trace.stderr_if(message, enabled);
+}
+
+/// Macro version for format string convenience
+///
+/// Usage: `log!(trace, enabled, "format {}", arg);`
+///
+/// This macro forwards to log_if() with format!() handling.
+#[macro_export]
+macro_rules! rewriter_log {
+    ($trace:expr, $enabled:expr, $($arg:tt)*) => {
+        $crate::mir::builder::control_flow::joinir::merge::rewriter::logging::log_if(
+            $trace,
+            $enabled,
+            &format!($($arg)*)
+        )
+    };
+}
+
+// Phase 260 P0.1: Keep local macro compatibility
+//
+// The original log! macro was defined locally in merge_and_rewrite().
+// This module provides both function (log_if) and macro (rewriter_log!) forms.
+//
+// Migration strategy:
+// 1. First, use logging::log_if() to replace some log! calls (verify)
+// 2. Then, replace remaining log! with rewriter_log! macro
+// 3. Finally, remove local log! macro from instruction_rewriter.rs

src/mir/builder/control_flow/joinir/merge/rewriter/mod.rs

@@ -18,14 +18,18 @@
 // Phase 260 P0.1 Step 1: Forward all declarations to parent instruction_rewriter.rs
 // This allows gradual migration without breaking existing code.
 //
-// When modules are ready:
-// - mod terminator;
-// - mod exit_line;
-// - mod carriers;
-// - mod logging;
+// Modules (extracted):
+// - logging: DEBUG-177 style verbose logs ✅
+
+// Future modules (pending):
+// - terminator: Branch/Jump/Return remapping
+// - exit_line: ExitLine/exit-phi wiring
+// - carriers: loop_invariants, exit_bindings
 //
 // For now, re-export from parent to maintain compatibility.
 
+pub(super) mod logging; // Phase 260 P0.1 Step 2: Logging extracted ✅
+
 // Re-export public API from parent instruction_rewriter module
 pub(super) use super::instruction_rewriter::is_skippable_continuation;
 pub(super) use super::instruction_rewriter::merge_and_rewrite;