Rename repository to bitcoin-script-lab

[polydeuces32]

Recommended manual repository rename:

Current:
bitcoin-script-playground

Target:
bitcoin-script-lab

Reason:
The project has been rebranded as Bitcoin Script Lab. The repository name should match the public product identity.

Manual steps:

1. Open repository Settings.
2. Under General, find Repository name.
3. Change bitcoin-script-playground to bitcoin-script-lab.
4. Click Rename.

GitHub will keep redirects from the old URL, but local remotes should be updated after rename:

git remote set-url origin https://github.com/polydeuces32/bitcoin-script-lab.git