Regular Cyclic $(q+1)$-Arcs in $\text{PG}(3,2^m)$: Spectral Rigidity, Descent, and an MDS Criterion

Let $q=2^m$ with $m\ge 3$ and set $n:=q+1$. We investigate $(q+1)$-arcs $\mathcal{A}\subset \mathrm{P}\mathrm{G}(3,q)$ that admit a regular cyclic subgroup $C\le \mathrm{P}\mathrm{G}\mathrm{L}(4,q)$ of order $n$. Over $K={\mathbb{F}}_{q^2}$, such an action can be conjugated to a diagonal one, producing explicit cyclic monomial models $${\mathcal{M}}_a=\{[1:t:t^a:t^{a+1}]:t\in U_n\}\subset \mathrm{P}\mathrm{G}(3,K),U_n=\{u\in K^{\times }:u^n=1\},$$ with $a\in (\mathbb{Z}\mathrm{/}n\mathbb{Z}{)}^{\times }$. We develop a spectral rigidity principle to obtain a precise descent criterion: ${\mathcal{M}}_a$ is $K$-projectively equivalent to a $(q+1)$-arc defined over ${\mathbb{F}}_q$ if and only if $a\equiv \pm 2^e(\mathrm{m}\mathrm{o}\mathrm{d}n)$ for some integer $e$ with $\gcd (e,m)=1$. Consequently, regular cyclic pairs $(\mathcal{A},C)$ fall into exactly $\phi (m)\mathrm{/}2$ $K$-projective equivalence classes. As an immediate coding-theoretic application, we resolve the remaining AMDS/MDS dichotomy for the BCH family ${\mathcal{C}}_{(q,q+1,3,h)}$ studied by Xu et al.: ${\mathcal{C}}_{(q,q+1,3,h)}$ is MDS if and only if $2h+1\equiv \pm 2^e(\mathrm{m}\mathrm{o}\mathrm{d}n)$ for some $e$ with $\gcd (e,m)=1$. The underlying spectral rigidity step is formulated in a general setting for diagonal regular cyclic pairs in $\mathrm{P}\mathrm{G}(r,K)$, providing a portable reduction of projective equivalence questions to explicit congruences on exponent data.