feat: warn when simp theorem LHS has variable or unrecognized head symbol

[wkrozowski]

This PR adds warnings when registering @[simp] theorems whose left-hand side has a problematic head symbol in the discrimination tree:

• Variable head (.star key): The theorem will be tried on every simp step, which can be expensive. The warning notes this may be acceptable for local or scoped simp lemmas. Controlled by warning.simp.varHead (default: true).
• Unrecognized head (.other key, e.g. a lambda expression): The theorem is unlikely to ever be applied by simp. Controlled by warning.simp.otherHead (default: true).

[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 2398d2cc66e3b981504e6066e60896476ff14a31 --onto 4f6bcc5adac8d6234a9e3def3675402cb89823c6. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-08 12:51:22)

[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 2398d2cc66e3b981504e6066e60896476ff14a31 --onto 861bc19e0c1c45b5766cc5c9ef0488568368b236. You can force reference manual CI using the force-manual-ci label. (2026-04-08 12:51:24)