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`).
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