fix the non-extendable DOL-language missing terms in WordMorphism

[cxzhong]

fix #40466
The Problem: The optimized algorithm assumes that all factors of length n in the D0L-language can be obtained by:

1. Taking 2-letter words from the language
2. Applying a high power of the morphism to each letter
3. Concatenating the images and extracting n-factors

What's Missing: This misses factors that:

Come directly from the axiom word u (like '000' from u='000')
Come from early iterations s^k(u) where k is small (like '010101' from m(000))
Are not extendable on both sides (non-extendable elements)

📝 Checklist

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