This PR fixes#12554 where the `cbv` tactic throws "unexpected kernel
projection term during structural definitional equality" when a rewrite
theorem's pattern contains a lambda and the expression being matched has
a `.proj` (kernel projection) at the corresponding position.
The `Sym` pattern matching infrastructure (`isDefEqMain` in
`Pattern.lean`) does not handle `.proj` expressions and can throw an
exception. Rather than presenting it as an error in `cbv`, we fail
quietly and let the `cbv` tactic try other fallback paths.
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
2ce55ba460