fix(ir): D12 — remove invented top-level error field on OpenAI response shapes

cold-audit catch from 2026-05-23

Cold-audit Finding 3 (P2 ALIGNMENT.md Rule 2(b) violation). The IR→OpenAI
translator was inventing a top-level `error: {message, type}` field on
chat.completion / chat.completion.chunk objects. OpenAI's spec defines
no such field — errors surface via HTTP 4xx/5xx with `{error: {...}}`
body, not as in-band SSE annotations. The pre-D12 code comments
acknowledged Rule 2(b) for `finish_reason` enum values then proceeded
to invent the top-level error field anyway.

Changes (3 files, +25 / -50 net):

1. lib/ir/ir-to-openai.mjs — both invention sites removed:
   - irChunkToOpenAISSE: removed `if (irChunk.type === 'error')` branch
     that emitted `{...spec fields, error: {message, type: 'provider_error'}}`
   - irResponseToOpenAINonStream: removed `else if (chunk.type === 'error')`
     branch + dead `errorChunk` local var + the `if (errorChunk)` block
     that appended top-level `response.error = {...}`
   - Added explanatory comments at both sites citing ALIGNMENT.md Rule 2(b)
     + OpenAI spec URLs so future readers see the rationale

2. server.mjs:581 — burst-replay loop break condition simplified:
   - Pre: `if (irChunk.type === 'stop' || irChunk.type === 'error') break;`
   - Post: `if (irChunk.type === 'stop') break;`
   - Cache writes (server.mjs:456, :470) only append to streamedChunks
     after the error-chunk guard fires, so cached chunks structurally
     cannot contain type === 'error'. The removed clause was dead.

3. test-features.mjs — one test rewritten (test count unchanged 288 → 288):
   - Old test asserted `payload.error.type === 'provider_error'` exists
     (i.e., it tested the violation as if it were correct)
   - New test asserts `payload.error === undefined` (the correct Rule 2(b)
     invariant) and that object stays `chat.completion.chunk` with
     finish_reason in the OpenAI enum

IR contract decision (verified by both implementer and reviewer):
- IR keeps `'error'` in IRResponseChunk typedef — providers legitimately
  emit error chunks (e.g., anthropic.mjs)
- server.mjs intercepts error chunks BEFORE translation:
  · collectAllChunks() throws ProviderError on type === 'error'
  · Real-streaming branch throws ProviderError (no first chunk) or
    truncates res.end() (after first chunk per ADR 0004 first-chunk rule)
- The translator is downstream of both guards; error chunks structurally
  cannot reach it in normal operation
- Therefore the invention sites were dead-code paths handling an
  impossible case — safe to remove
- Pass-through behavior on the impossible path: produces an empty SSE
  delta `{choices: [{index: 0, delta: {}, finish_reason: null}]}`
  (verified manually by reviewer); the error message text does NOT
  leak to the wire

288/288 tests pass on Node 20.20.2.

Authority:
- ALIGNMENT.md Rule 2(b) — no invented OpenAI-spec fields
- OpenAI Chat Completions object spec
  https://platform.openai.com/docs/api-reference/chat/object
  https://platform.openai.com/docs/api-reference/chat/streaming
- ADR 0004 § Fallback safety — first-chunk rule (preserves the
  truncation semantics for post-first-chunk errors)

Reviewer (Iron Rule v1.6 § 10.x Mode A, fresh-context opus, independent
of drafter): APPROVE. Verified error chunks never reach translator via
three independent grep passes (collectAllChunks guard, real-streaming
guard, no third caller exists), verified cache cannot contain error
chunks (streamedChunks.push is post-guard), verified Rule 2(b) compliance
by direct read of ALIGNMENT.md line 29 + OpenAI spec citations.

Three non-blocking suggestions (defensive log on impossible path;
defensive guard at server.mjs:587; comment placement nit) explicitly
"Not for D12" per the reviewer — preserved as separate cleanup
opportunities if ever needed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server.mjs

@@ -578,7 +578,7 @@ async function handleChatCompletions(req, res) {
 
     for (const irChunk of chunks) {
       res.write(irChunkToOpenAISSE(irChunk, requestId, ir.model));
-      if (irChunk.type === 'stop' || irChunk.type === 'error') break;
+      if (irChunk.type === 'stop') break;
     }
     res.write(SSE_DONE);
     res.end();