feat: make Instance.create optional to fix type compatibility

[konard]

Summary

This PR fixes issue #43 by making the create method optional on the Instance type, which allows pool-returned instances to be properly typed as Instance.

Problem

Previously, users couldn't type their code properly when using the Pool:

import { definePool, Pool, Instance } from 'prool';

let instance: Instance;
instance = await pool.start(1);  // ❌ Type error!

This error occurred because:

• The Pool internally used Instance_ type (defined as Omit<Instance, 'create'>)
• Pool methods like start() returned Instance_ instead of Instance
• Instance_ was not assignable to Instance due to the missing create property

Solution

Instead of using the internal Instance_ type, we:

1. Made create optional in the Instance type definition (create? instead of create)
2. Removed the Instance_ internal type from Pool.ts entirely
3. Updated Pool to use Instance type directly for all its methods
4. Added runtime check in Pool.start() to ensure the instance has a create method when creating new instances
5. Updated return type of create method to return Instance<_internal> instead of Omit<Instance<_internal>, 'create'>

After this change

import { definePool, Pool, Instance } from 'prool';

let instance: Instance;
instance = await pool.start(1);  // ✅ Works!

The Instance type now properly represents both:

• Direct instances (which have the create method)
• Pool-returned instances (which don't have the create method)

Test plan

• TypeScript type checking passes (pnpm check:types)
• Linting passes (pnpm check)
• Manual verification that the type error from issue Remove Instance_ internal type #43 is resolved

Fixes #43

🤖 Generated with Claude Code

[konard]

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[konard]

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