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/startup_profiler.py

@@ -37,8 +37,8 @@ class StartupProfiler:
    try:
     sys.stderr.write(f"[startup] {name}: {(p.end_ts - p.start_ts) * 1000.0:.1f}ms\n")
     sys.stderr.flush()
-   except OSError:
-    pass
+   except OSError as e:
+    sys.stderr.write(f"[startup] phase output failed for {name}: {e}\n")
 
  def snapshot(self) -> dict[str, Any]:
   phases: dict[str, dict[str, float]] = {}