refactor(query): deepen dispatch policy seam with structured result contract (#3066)

* refactor(query): deepen dispatch policy seam with structured result contract

Closes #3065.

- unify query dispatch outcome as typed success/failure union
- include error kind/details + final exit_code in failure path
- align native and fallback paths under one dispatch policy seam
- make CLI query path consume seam result (thin adapter)
- add ADR + context term for Dispatch Policy Module

* refactor(query): strengthen dispatch seam with shared error mapper and typed details

- add query-dispatch-error-mapper module shared by native/fallback paths
- remove ad-hoc inline mapping in dispatch/fallback executors
- lock error-details schema in mapper + dispatch tests
- document structured dispatch contract in QUERY-HANDLERS.md

* fix(query): return structured fallback failure when path resolution throws

- guard resolveGsdToolsPath in cjs dispatch path
- map thrown resolution errors to fallback_failure result
- add regression test for structured failure contract

sdk/src/query/query-dispatch-error-mapper.ts

@@ -0,0 +1,51 @@
+import type { QueryDispatchError, QueryDispatchErrorKind, QueryDispatchResult } from './query-dispatch-contract.js';
+
+export function toDispatchFailure(
+  error: QueryDispatchError,
+  stderr: string[] = [],
+): QueryDispatchResult {
+  return {
+    ok: false,
+    stderr,
+    exit_code: error.code,
+    error,
+  };
+}
+
+export function mapNativeDispatchError(error: unknown, command: string, args: string[]): QueryDispatchError {
+  const message = error instanceof Error ? error.message : String(error);
+  const kind: QueryDispatchErrorKind = message.includes('timed out after')
+    ? 'native_timeout'
+    : 'native_failure';
+  return {
+    kind,
+    code: 1,
+    message: `Error: ${message}`,
+    details: {
+      command,
+      args,
+      ...(kind === 'native_timeout' ? { timeout_ms: parseTimeoutMs(message) } : {}),
+    },
+  };
+}
+
+export function mapFallbackDispatchError(error: unknown, command: string, args: string[]): QueryDispatchError {
+  const message = error instanceof Error ? error.message : String(error);
+  return {
+    kind: 'fallback_failure',
+    code: 1,
+    message: `Error: gsd-tools.cjs fallback failed: ${message}`,
+    details: {
+      command,
+      args,
+      backend: 'cjs',
+    },
+  };
+}
+
+function parseTimeoutMs(message: string): number | undefined {
+  const m = message.match(/timed out after\s+(\d+)ms/i);
+  if (!m) return undefined;
+  const n = Number.parseInt(m[1], 10);
+  return Number.isFinite(n) ? n : undefined;
+}