fix: handle non-internalized terms in semiring reifier

This PR fixes a panic in `grind` where `sreifyCore?` could encounter
power subterms not yet internalized in the E-graph during nested
propagation. The ring reifier (`reifyCore?`) already had a defensive
`alreadyInternalized` check before creating variables, but the semiring
reifier (`sreifyCore?`) was missing this guard. When `propagatePower`
decomposed `a ^ (b₁ + b₂)` into `a^b₁ * a^b₂` and the resulting terms
triggered further propagation, the semiring reifier could be called on
subterms not yet in the E-graph, causing `markTerm` to fail.

Closes #12428

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>