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We investigate threshold phenomena in dissipative quantum systems through the spectral theory of the Liouvillian generator. Building on Band 42.9+0 of the Resonant Boundary Series — which identified a sweet spot σ* ≈ 0.941 of maximal persistence in a Lindblad array — this work provides the formal spectral foundations of that observation. The central result is a formal identification between three equivalent definitions of the sweet spot: the maximum of the effective memory time τ_eff(σ), the minimum of the Liouvillian gap Δ_L(σ) = −Re(λ_2(σ)), and the maximum of the von Neumann entropy plateau S_plateau(σ). This triple equivalence — σ* = argmax τ_eff = argmin Δ_L = argmax S_plateau — constitutes the spectral backbone of the Resonant Boundary framework. We introduce the concept of spectral memory as the persistence of initial-state coefficients c_n(0) within the metastable manifold M = span{R_n : |Re(λ_n)| ≪ 1/t_obs}. The slow eigenmodes of ℒ, which we term Schattenkinder (shadow children) in the series’ narrative language, become accessible near σ*. Their formal signature is a slow cluster of eigenvalues with Re(λ_n) ≈ 0 near the imaginary axis. The Flickering Transition — a stochastic switching between coexisting attractors near σ* — is formalized through the rate Γ_{A→B} ∝ exp(−N·ΔF_eff) and characterized by the oscillating-mode gap Δ_OM as a Flickering thermometer. We propose the Double Gap Closing hypothesis: both Δ_L and Δ_OM reach their minimum simultaneously at σ*. The present work proposes a universal resonance condition (Definition 1) linking structural persistence, spectral gaps, and boundary dynamics: σ* = argmin Δ_L = argmax τ_eff = argmax S_plateau Structural isomorphisms to six independent domains are identified: THz-photonic metasurfaces (bound states in the continuum, BIC), neuronal criticality (edge of chaos), climate tipping points (critical slowing down), acoustic avoided crossings, Friedrich-Wintgen BIC interference, and machine learning phase transitions (grokking). All mappings are treated as structural isomorphisms, not claims of physical equivalence. The work is a collaborative preprint by a human researcher and five AI systems (ChatGPT/OpenAI, Gemini/Google, Claude AI/Anthropic, Ratta/Grok/xAI, Consensus AI), continuing the open research programme of the Resonant Boundary Series. Wir untersuchen Schwellenphänomene in dissipativen Quantensystemen mittels der Spektraltheorie des Liouvillian-Generators. Aufbauend auf Band 42.9+0 der Resonant Boundary Series — der bei σ* ≈ 0.941 ein Maximum der Persistenzzeit τ_eff in einem Lindblad-Array identifiziert hat — liefert dieser Band die formale Spektralgrundlage. Kernresultat (Definition 1): drei äquivalente Definitionen des Sweet-Spots σ* = argmax τ_eff = argmin Δ_L = argmax S_plateau als universelle Resonanzbedingung. Das Konzept des spektralen Gedächtnisses wird eingeführt. Die Flickering Transition wird formal verankert. Strukturelle Isomorphismen zu THz-Photonik, Neurologie, Klimasystemen und KI-Architektur werden identifiziert und als Werkzeuge epistemischer Zurückhaltung behandelt.