evals: preflight ping once in the sharded parent, not per shard

The Anthropic fail-fast ping ran at module load in every paid test file
importing e2e-helpers — ~30 paid claude -p calls (30s timeout each) per
full sharded run for one bit of information. The parent now pings once
before spawning shards and sets EVALS_PREFLIGHT_OK=1; the module-load
path honors the flag. Extracted to test/helpers/anthropic-preflight.ts
(injectable spawn seam) with regression pins in both directions: the
flag must skip, its absence must ping exactly once, dead API must throw.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/anthropic-preflight.test.ts

@@ -0,0 +1,53 @@
+/**
+ * Regression pins for the preflight-dedup seam (test/helpers/anthropic-preflight.ts).
+ *
+ * The preflight runs at MODULE LOAD in every paid test file that imports
+ * e2e-helpers, so a broken skip-check either brings back ~30 paid pings per
+ * sharded run or — worse — skips the fail-fast everywhere. Both directions
+ * are pinned here with an injected spawn; no real claude call is made.
+ */
+
+import { describe, test, expect } from 'bun:test';
+import { preflightAnthropicApi } from './helpers/anthropic-preflight';
+
+type SpawnCall = { command: string; args: string[] };
+
+function fakeSpawn(stdout: string, calls: SpawnCall[]) {
+  return ((command: string, args: string[]) => {
+    calls.push({ command, args });
+    return { stdout: Buffer.from(stdout), stderr: Buffer.from(''), status: 0 } as ReturnType<
+      typeof import('child_process').spawnSync
+    >;
+  }) as typeof import('child_process').spawnSync;
+}
+
+describe('preflightAnthropicApi', () => {
+  test('EVALS_PREFLIGHT_OK=1 skips the ping entirely (sharded-child path)', () => {
+    const calls: SpawnCall[] = [];
+    const result = preflightAnthropicApi({ EVALS_PREFLIGHT_OK: '1' }, fakeSpawn('never read', calls));
+    expect(result).toBe('skipped');
+    expect(calls.length).toBe(0);
+  });
+
+  test('without the flag, pings exactly once and passes on healthy output', () => {
+    const calls: SpawnCall[] = [];
+    const result = preflightAnthropicApi({}, fakeSpawn('{"type":"result"}', calls));
+    expect(result).toBe('ok');
+    expect(calls.length).toBe(1);
+    expect(calls[0].args.join(' ')).toContain('claude -p');
+  });
+
+  test('unreachable API throws (fail-fast before any shard spawns)', () => {
+    const calls: SpawnCall[] = [];
+    expect(() =>
+      preflightAnthropicApi({}, fakeSpawn('error: ConnectionRefused connecting to api', calls)),
+    ).toThrow(/Anthropic API unreachable/);
+  });
+
+  test('a truthy-but-not-"1" flag still pings (no accidental widening)', () => {
+    const calls: SpawnCall[] = [];
+    const result = preflightAnthropicApi({ EVALS_PREFLIGHT_OK: 'true' }, fakeSpawn('ok', calls));
+    expect(result).toBe('ok');
+    expect(calls.length).toBe(1);
+  });
+});