feat: Thunk is inhabited

[JovanGerb]

This PR adds the Inhabited instance for Thunk.

We need this in batteries to call PersistentEnvExtension.getState on a state that is wrapped in a Thunk, see https://github.com/leanprover-community/batteries/pull/1667/changes.

[JovanGerb]

changelog-library

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 3c64f6a74934b832f9578be117052c5e29c80f39 --onto 6ca23a7b8bee57152110ce500ce795148707d4ed. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-13 17:44:18)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 3c64f6a74934b832f9578be117052c5e29c80f39 --onto 9da8f5dce42832b8411d58446653f3640a94a6e3. You can force reference manual CI using the force-manual-ci label. (2026-02-13 17:44:20)

[JovanGerb]

Should I just make an empty commit to make CI green, now that I've added the changelog label?

[nomeata]

One way to get CI to notice the label is to close and open the PR. Empty commit or force push work too.

[Vtec234]

Mathlib build succeeded here: #12991.