fix: getUsedConstants should record projection type names

[hargoniX]

This PR makes Expr.getUsedConstants collect the typeName field of Expr.proj so we get a full list of constants that are directly used.

[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 f7f5b4cda9b33387b512b84ea179461881b12500 --onto b1cd183c9978c3bedddc33baf7c9599baaeca046. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-10 09:09:18)

[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 f7f5b4cda9b33387b512b84ea179461881b12500 --onto b1cd183c9978c3bedddc33baf7c9599baaeca046. You can force reference manual CI using the force-manual-ci label. (2026-08-10 09:09:20)