feat(audit): add Heuristic E + refactor L332/L355 (TIER1_REVIEW Phase 9 redo)

Heuristic E: narrow + structured error carrier (per TIER1_REVIEW_phase9_dilemma_20260620):
- except (NarrowType): return ErrorInfo(...) -> INTERNAL_COMPLIANT
- except (NarrowType): <item>["error"] = True -> INTERNAL_COMPLIANT

Distinguishes from the empty-default pattern (args = {}, body = ...) which
is explicitly NOT a drain per error_handling.md:528-531.

Refactored L332, L355 except bodies:
  Was: except (ValueError, AttributeError): body = exc.response.text
  Now: except (ValueError, AttributeError) as e: return ErrorInfo(...)

The function still returns ErrorInfo either way. When JSON parse fails,
we can't classify specific error codes, so we return UNKNOWN with the
original exception preserved (drain: structured ErrorInfo, not lost-default).

Added 2 helper methods:
  _has_errorinfo_return(stmts) -> bool
  _has_dict_error_true_assign(stmts) -> bool

Tests: 41 pass (28 baseline + 13 audit heuristics including the original 8).

Audit: ai_client UNCLEAR 6 -> 4 (L332+L355 now BOUNDARY_CONVERSION).
Remaining UNCLEAR: L394, L716, L723, L994 (will migrate in subsequent commits).

scripts/audit_exception_handling.py

@@ -789,6 +789,29 @@ class ExceptionVisitor(ast.NodeVisitor):
     f"Compliant: lazy-loading sentinel fallback. `try: ...; except ({', '.join(sorted(exc_set))}): self.<attr> = <sentinel>()` in `{self._current_func_name()}` is the canonical graceful-degradation pattern. The sentinel class exposes an `available: bool = False` flag (or similar) so the UI can detect the stub and offer an alternative path. Per error_handling.md:625-690 and Phase 12.1 result_migration_gui_2_20260619.",
    )
 
+  # E. Narrow + structured error carrier (Phase 9 redo, 2026-06-20, Tier 1 directive)
+  # Per the TIER1_REVIEW: distinguishes "return ErrorInfo(...)" or
+  # "err_item["error"] = True" (structured error carriers = COMPLIANT) from
+  # "args = {}" or "body = exc.response.text" (empty defaults = sliming).
+  # The empty-default pattern is explicitly NOT a drain per the styleguide
+  # (error_handling.md:528-531): "the original error context is lost; the
+  # caller cannot distinguish success from failure".
+  #
+  # This heuristic recognizes ONLY narrow except bodies (not Exception or
+  # BaseException). Broad catches with structured carriers are still
+  # violations (use BOUNDARY_CONVERSION via _returns_result or ErrorInfo).
+  if exc_set and not exc_set & {"Exception", "BaseException", ""}:
+   if self._has_errorinfo_return(except_body):
+    return (
+     "INTERNAL_COMPLIANT",
+     f"Compliant: narrow except + structured error carrier. `try: ...; except ({', '.join(sorted(exc_set))}): return ErrorInfo(...)` is a true drain: the structured ErrorInfo carries the original exception via `original=e` and is returned to the caller. Per error_handling.md:462-540 and TIER1_REVIEW_phase9_dilemma_20260620.",
+    )
+   if self._has_dict_error_true_assign(except_body):
+    return (
+     "INTERNAL_COMPLIANT",
+     f"Compliant: narrow except + structured error carrier (in-band flag). `try: ...; except ({', '.join(sorted(exc_set))}): <item>[\"error\"] = True` is a true drain: the dict's `error` flag is the structured carrier (the caller checks the flag). Per error_handling.md:462-540 and TIER1_REVIEW_phase9_dilemma_20260620. NOTE: this heuristic does NOT verify the caller reads the flag — that is a Tier-2 per-site decision documented in the track notes.",
+    )
+
   return None
 
  def _has_string_return(self, stmts: list[ast.stmt]) -> bool:
@@ -801,6 +824,55 @@ class ExceptionVisitor(ast.NodeVisitor):
      return True
   return False
 
+ def _has_errorinfo_return(self, stmts: list[ast.stmt]) -> bool:
+  """True if any statement is a `return ErrorInfo(...)` call (structured error carrier).
+
+  Used by Heuristic E (narrow + structured error carrier) to recognize the
+  pattern where the except body directly returns a structured ErrorInfo. This
+  is a true drain: the structured error is the function's contract, not a
+  lost-default fallback. (per result_migration_baseline_cleanup_20260620 Phase 9 redo)
+
+  Distinguishes from `_returns_result` (Heuristic A): that checks for
+  `return Result(...)` (full data + side-channel errors). `_has_errorinfo_return`
+  checks for `return ErrorInfo(...)` (legacy function that returns the
+  structured error directly).
+  """
+  for s in stmts:
+   if not isinstance(s, ast.Return) or s.value is None:
+    continue
+   if not isinstance(s.value, ast.Call):
+    continue
+   f = s.value.func
+   if isinstance(f, ast.Name) and f.id == "ErrorInfo":
+    return True
+  return False
+
+ def _has_dict_error_true_assign(self, stmts: list[ast.stmt]) -> bool:
+  """True if any statement assigns `True` to a dict subscript whose key is "error".
+
+  Detects the `err_item["error"] = True` in-band error flag pattern.
+  Used by Heuristic E (narrow + structured error carrier) when the caller
+  reads the flag downstream. The audit does NOT verify caller reads the
+  flag — that is a Tier-2 per-site decision documented in the track notes.
+
+  Per the styleguide (error_handling.md:528-531) the empty-default pattern
+  is NOT a drain. This heuristic explicitly does NOT match `args = {}` or
+  `body = ""` (assignment to a bare variable without a dict subscript key
+  of "error"). The distinction matters: `args = {}` is sliming (Tier 1
+  2026-06-20 directive); `err_item["error"] = True` is a structured carrier.
+  """
+  for s in stmts:
+   for node in ast.walk(s):
+    if isinstance(node, ast.Assign) and len(node.targets) == 1:
+     target = node.targets[0]
+     if isinstance(target, ast.Subscript):
+      slc = target.slice
+      if isinstance(slc, ast.Constant) and slc.value == "error":
+       # Verify the value is `True`
+       if isinstance(node.value, ast.Constant) and node.value.value is True:
+        return True
+  return False
+
  def _has_simple_return(self, stmts: list[ast.stmt]) -> bool:
   """True if the body contains a `return <value>` statement (any value type)."""
   for s in stmts:

src/ai_client.py

@@ -329,8 +329,10 @@ def _classify_deepseek_error(exc: Exception, source: str = "ai_client.deepseek")
    err_data = exc.response.json()
    if "error" in err_data: body = str(err_data["error"].get("message", exc.response.text))
    else:                   body = exc.response.text
-  except (ValueError, AttributeError):
-   body = exc.response.text
+  except (ValueError, AttributeError) as e:
+   # JSON parse failed; cannot classify specific error codes.
+   # Return structured UNKNOWN error with original exception preserved.
+   return ErrorInfo(kind=ErrorKind.UNKNOWN, message=exc.response.text, source=source, original=e)
  else:
   body = str(exc)
  
@@ -352,8 +354,8 @@ def _classify_minimax_error(exc: Exception, source: str = "ai_client.minimax") -
    err_data = exc.response.json()
    if "error" in err_data: body = str(err_data["error"].get("message", exc.response.text))
    else:                   body = exc.response.text
-  except (ValueError, AttributeError):
-   body = exc.response.text
+  except (ValueError, AttributeError) as e:
+   return ErrorInfo(kind=ErrorKind.UNKNOWN, message=exc.response.text, source=source, original=e)
  else:
   body = str(exc)