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Status: exact $E_{47}$ projector combined with verified HBr constants and an exploratory numerical routing construction.
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The molecular side supplies measured or tabulated HBr constants. Independently, the canonical finite-dimensional operator is
$$ K=(C-6I)(C-30I) $$
on
$$ V_2^{\otimes3}. $$
Let the columns of $N\in\mathbb C^{125\times47}$ form an orthonormal basis of $\ker K$. Then
$$ P_E=NN^\dagger $$
satisfies
$$ P_E^2=P_E, $$
$$ P_E^\dagger=P_E, $$
$$ KP_E=0, $$
and
$$ \operatorname{Tr}(P_E)=47. $$