fix(web): close Heid pass-2 findings — stream vocab + disconnect catch (v0.16.1)

Second Heid panel pass (thread 01KSPBMFRRQE) on the v0.16.0 tree:
Gróa returned zero findings; Hulda surfaced two minor tightening
items, both closed here.

1. test-gap — TestStreamFullEventVocab drove only 8 of 11 Event types
   through the stream endpoint (omitted Error, Cancelled, AffectUpdate).
   Serialization for all 11 was already covered by the presentation-
   contract fixture tests; this was a stream-integration coverage gap.
   - Added AffectUpdate to the vocab stream (non-terminal, coexists
     with done).
   - Added dedicated test_error_terminal_event + test_cancelled_terminal_event
     (terminal events are mutually exclusive with done, so they can't
     share one stream).

2. precision — the disconnect-cancel path caught bare `except Exception:
   pass`, silently swallowing real CancelFailed / transport errors. The
   contract intent is to swallow only the cooperative race
   (CancelAlreadyCompleted). Narrowed: swallow CancelAlreadyCompleted /
   CancelTurnNotFound as the no-op race; log unexpected cancel failures
   as a structured stderr line for diagnosability. Never re-raises (we're
   unwinding the cancelled generator and must not mask CancelledError).

Tests: +2 (376 → 378). Patch per SemVer discipline — coverage +
diagnosability tightening, no behavior change observable to callers.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.16.0"
+version = "0.16.1"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"