auditor + gateway: 2 fixes from kimi_architect's first real run

Acted on 2 of 10 findings Kimi caught when auditing its own integration
on PR #11 head 8d02c7f. Skipped 8 (false positives or out-of-scope).

1. crates/gateway/src/v1/kimi.rs — flatten OpenAI multimodal content
   array to plain string before forwarding to api.kimi.com. The Kimi
   coding endpoint is text-only; passing a [{type,text},...] array
   returns 400. Use Message::text() to concat text-parts and drop
   non-text. Verified with curl using array-shape content: gateway now
   returns "PONG-ARRAY" instead of upstream error.

2. auditor/checks/kimi_architect.ts — computeGrounding switched from
   readFileSync to async readFile inside Promise.all. Doesn't matter
   at 10 findings; would matter at 100+. Removed unused readFileSync
   import.

Skipped findings (with reason):
- drift_report.ts:18 schema bump migration concern: the strict
  schema_version refusal IS the migration boundary (v1 readers
  explicitly fail on v2; not a silent corruption risk).
- replay.ts:383 ISO timestamp precision: Date.toISOString always
  emits "YYYY-MM-DDTHH:mm:ss.sssZ" (ms precision). False positive.
- mode.rs:1035 matrix_corpus deserializer compat: deserialize_string
  _or_vec at mode.rs:175 already accepts both shapes. Confabulation
  from not seeing the deserializer in the input bundle.
- /etc/lakehouse/kimi.env world-readable: actually 0600 root. Real
  concern would be permission-drift; not a code bug.
- callKimi response.json hang: obsolete; we use curl now.
- parseFindings silent-drop: ergonomic concern, not a bug.
- appendMetrics join with "..": works for current path; deferred.
- stubFinding dead-type extension: cosmetic.

Self-audit grounding rate at v1.0.0: 10/10 file:line citations
verified by grep. 2 of 10 actionable bugs landed. The other 8 were
correctly flagged as concerns but didn't earn a code change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

auditor/checks/kimi_architect.ts

@@ -27,7 +27,7 @@
 // Off by default: caller checks LH_AUDITOR_KIMI=1 before invoking.
 
 import { readFile, writeFile, mkdir, appendFile, stat } from "node:fs/promises";
-import { existsSync, readFileSync } from "node:fs";
+import { existsSync } from "node:fs";
 import { join, resolve } from "node:path";
 import type { Finding, CheckKind } from "../types.ts";
 
@@ -265,31 +265,36 @@ function parseFindings(content: string): Finding[] {
 // is appended into the evidence array so the reader can see which
 // citations were verified.
 async function computeGrounding(findings: Finding[]): Promise<{ total: number; verified: number; rate: number }> {
-  let verified = 0;
-  for (const f of findings) {
+  // readFile (async) instead of readFileSync — caught 2026-04-27 by
+  // Kimi's self-audit. Sync I/O in an async fn blocks the event loop
+  // for every cited file; doesn't matter at 10 findings, would matter
+  // at 100+.
+  const checks = await Promise.all(findings.map(async (f) => {
     const cite = f.evidence[0] ?? "";
     const m = /^(\S+?):(\d+)/.exec(cite);
-    if (!m) continue;
+    if (!m) return false;
     const [, relpath, lineStr] = m;
     const line = Number(lineStr);
-    if (!line || !relpath) continue;
+    if (!line || !relpath) return false;
     const abs = relpath.startsWith("/") ? relpath : resolve(REPO_ROOT, relpath);
     if (!existsSync(abs)) {
       f.evidence.push("[grounding: file not found]");
-      continue;
+      return false;
     }
     try {
-      const lines = readFileSync(abs, "utf8").split("\n");
+      const lines = (await readFile(abs, "utf8")).split("\n");
       if (line < 1 || line > lines.length) {
         f.evidence.push(`[grounding: line ${line} > EOF (${lines.length})]`);
-        continue;
+        return false;
       }
       f.evidence.push(`[grounding: verified at ${relpath}:${line}]`);
-      verified++;
+      return true;
     } catch (e) {
       f.evidence.push(`[grounding: read failed: ${(e as Error).message.slice(0, 80)}]`);
+      return false;
     }
-  }
+  }));
+  const verified = checks.filter(Boolean).length;
   const total = findings.length;
   return { total, verified, rate: total === 0 ? 0 : verified / total };
 }