This PR replaces eager let-expression zeta-reduction in the sym-based
mvcgen with on-demand unfolding that mirrors the production mvcgen's
behavior.
Previously, all let-expressions in the program head were immediately
zeta-reduced. Now, let-expressions are hoisted to the top of the goal
target, and the value is only inlined if it is duplicable (literals,
fvars, consts, `OfNat.ofNat`). Complex values are introduced into the
local context via `introsSimp`, preserving SymM's maximal sharing
invariants, and unfolded on demand when the fvar later appears as the
program head.
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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