refactor(src): Phase 10.2 batch 6 - hot_reloader + warmup + startup_profiler

hot_reloader.py (1 site - module reload with broad except):
- reload() returns Result[bool] now. The migration catches the
  broad Exception, captures it as ErrorInfo with the traceback in
  last_error, and returns Result(data=False, errors=[...]).
- reload_all() returns Result[bool]; aggregates per-module errors.
- The class still tracks last_error and is_error_state for
  backwards-compat with any caller reading the class attributes.

warmup.py (5 sites):
- L139 (on_complete callback fire): was except ...: pass.
  Now logs to sys.stderr with the exception.
- L215 (_record_success callback fire): same.
- L249 (_record_failure callback fire): same.
- L276 (_log_canary stderr.write): was except OSError: pass.
  Now logs the OSError itself.
- L300 (_log_summary stderr.write): same.

startup_profiler.py (1 site - context manager):
- phase() is a context manager (yields); can't return Result.
  The except inside the finally block now logs the OSError.

Tests updated for hot_reloader to check result.ok and result.data.

Tests verified:
- tests/test_hot_reloader.py (9 tests) PASS
- tests/test_hot_reload_integration.py (13 tests) PASS
- tests/test_warmup.py (10 tests) PASS
- tests/test_warmup_canaries.py (18 tests) PASS

src/warmup.py

@@ -136,8 +136,8 @@ class WarmupManager:
   if fire_now:
    try:
     callback(snap)
-   except Exception:
-    pass
+   except Exception as e:
+    sys.stderr.write(f"[WarmupManager] on_complete callback raised: {e}\n")
 
  def reset(self) -> None:
   with self._lock:
@@ -212,8 +212,8 @@ class WarmupManager:
   for cb in callbacks:
    try:
     cb(self._snapshot())
-   except Exception:
-    pass
+   except Exception as e:
+    sys.stderr.write(f"[WarmupManager] _record_success callback raised: {e}\n")
   if all_done:
    self._done_event.set()
 
@@ -246,8 +246,8 @@ class WarmupManager:
   for cb in callbacks:
    try:
     cb(self._snapshot())
-   except Exception:
-    pass
+   except Exception as e:
+    sys.stderr.write(f"[WarmupManager] _record_failure callback raised: {e}\n")
   if all_done:
    self._done_event.set()
 
@@ -273,7 +273,8 @@ class WarmupManager:
    try:
     sys.stderr.write(line)
     sys.stderr.flush()
-   except OSError: pass
+   except OSError as e:
+    sys.stderr.write(f"[WarmupManager] canary log write failed: {e}\n")
 
  def _log_summary(self) -> None:
   if not self._log_to_stderr: return
@@ -297,7 +298,8 @@ class WarmupManager:
      if main_thread_violations:
       sys.stderr.write(f"[warmup WARNING] {len(main_thread_violations)} module(s) loaded on the MAIN THREAD (violates main thread purity invariant): {', '.join(main_thread_violations)}\n")
      sys.stderr.flush()
-    except OSError: pass
+    except OSError as e:
+     sys.stderr.write(f"[WarmupManager] summary log write failed: {e}\n")
 
  def _snapshot(self) -> dict[str, list[str]]:
   return {