feat(invariants): implement package runtime checks

packages/spill/spill-policy/tests/spill-policy.spec.ts

@@ -109,6 +109,17 @@ describe('config validation', () => {
   it('rejects a fractional maxInlineBytes at load', async () => {
     await expect(setup({ maxInlineBytes: 1.5 })).rejects.toThrow(/non-negative integer/)
   })
+
+  it('rejects a configured policy that omits its post-execute listener', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SystemPrompt)
+    await ctx.plugin(ToolRegistry)
+    await expect(ctx.plugin({
+      name: 'spill-policy',
+      inject: ['tools'],
+      apply(_child: Context, _config: { maxInlineBytes?: number }) {},
+    }, { maxInlineBytes: 10 })).rejects.toThrow(/listener must exist exactly when maxInlineBytes is configured/)
+  })
 })
 
 describe('oversized plain-text replacement', () => {