Problem. Let \(e_1=(1,0)\), \(e_2=(0,1)\), and let \(\|\cdot\|_2\) denote the fixed coordinate Euclidean norm, used only as an external coordinate-error gauge and in the conclusion to be reconstructed. Let \(\ell:\mathbb R^2\to[0,\infty)\) be continuous, positive away from \(0\), even, and positively homogeneous, with exact axial calibration \(\ell(se_1)=\ell(se_2)=|s|\) for every \(s\in\mathbb R\). Let \(G=\{R_t:t\in\mathbb R\}\) be a continuous one-parameter subgroup of \(GL(2,\mathbb R)\) whose induced action on rays through the origin is transitive. Assume that for some \(0\le \varepsilon<1\), \[(1-\varepsilon)\ell(v)\le \ell(R_t v)\le(1+\varepsilon)\ell(v)\] for every \(v\in\mathbb R^2\) and every \(t\in\mathbb R\). Normalize the group parameter so that a distinguished time \(\pi/2\) is specified, and define the coordinate quarter-turn defect \[\delta=\max\{\|R_{\pi/2}e_1-e_2\|_2,\ \|R_{\pi/2}e_2+e_1\|_2\}.\] Define the normalized Pythagorean defect \[D(\ell)=\sup_{v\ne0}\frac{|\ell(v)^2-\|v\|_2^2|}{\|v\|_2^2}.\] Is there a universal constant \(C<\infty\) and \(\eta_0>0\) such that \[D(\ell)\le C(\varepsilon+\delta)\] whenever \(\varepsilon+\delta\le\eta_0\)? If yes, determine the sharp first-order stability constant \[C_* = \limsup_{\eta\to0^+}\ \sup\left\{\frac{D(\ell)}{\varepsilon+\delta}: (\ell,G)\text{ admissible},\ 0<\varepsilon+\delta\le\eta\right\}.\] If no, construct an explicit sequence of admissible \((\ell_n,G_n)\) with \(\varepsilon_n+\delta_n\to0\) but \(D(\ell_n)\) bounded away from zero. Also determine which of continuity, evenness, positive homogeneity, uniform-in-\(t\) approximate invariance, ray transitivity, and two-axis calibration are essential by giving countermodels whenever an assumption is claimed essential. No inner-product representation or Cartesian distance formula for \(\ell\) may be assumed.