fix: allow Child() to accept relative 'kind/id' path (backwards compatible)

[ignoxx]

Problem

Currently, ctx.Child() requires the full PID.ID (e.g., "parent/1/worker/123") to find a child actor. This is counter-intuitive because:

1. SpawnChild(producer, "worker", WithID("123")) accepts kind and id separately
2. Users naturally expect ctx.Child("worker/123") to work when called from the parent actor
3. The current behavior requires users to manually construct the full path

Related issue: #168

Solution

This PR modifies Child() to first try an exact match (for backward compatibility), then try prepending the parent's PID.ID to support the intuitive "kind/id" format.

Usage

// Spawn a child
ctx.SpawnChild(producer, "worker", WithID("123"))
// Now both work:
ctx.Child("worker/123")                    // ✅ New intuitive way
ctx.Child("parent/1/worker/123")           // ✅ Still works (backward compatible)

[ignoxx]

@tralafiti @anthdm would be nice to have this merged :D