Observers in quantum field theory cannot be fundamental excitations

Quantum field theory unifies quantum mechanics with special relativity, so any physical system playing the observer role in a QFT setting must simultaneously satisfy the SR-observer condition (a Poincaré-covariant worldline-tetrad bundle at the representation level) and the QM-observer condition (persistent macroscopic pointer structure supporting record formation). We take the intersection of these two conditions as the necessary content of "QFT observer" and apply it to the fundamental quantum field excitations of the Standard Model. Massless helicity states (photon; gluon perturbatively; hypothetical asymptotic graviton) fail the SR-observer condition because every four-momentum in the spectral support is null and no construction on the massless helicity representation supplies a mode-level rest-frame bundle. Newton-Wigner localisation is excluded on the massless helicity-±1 representation under the standard Wightman hypotheses, non-Wightman photon-position constructions each relax at least one requirement of the bundle, the Fokker-Pryce centroid is singular in the massless limit, and Ehrenfest expectation-value trajectories on non-Wightman photon-position operators fail the mode-level rest-frame requirement even when their tangent is timelike. Massive fundamental bosons (Higgs, W, Z) satisfy the SR-observer condition on their spectral support but fail the QM-observer condition because their decay widths preclude persistent macroscopic structure over any operationally relevant interval (Higgs τ ~ 10⁻²² s; W and Z τ ~ 10⁻²⁵ s, versus a macroscopic detector coarse-graining scale σ ≥ 10⁻¹² s) and the Standard Model contains no stable bound composites of pure fundamental bosons. The elimination leaves fermionic composites as the only surviving class of candidate QFT observer within the Standard Model. The positive characterisation of that class, including the specific field-theoretic content playing the observer role and the application to decoherence branch-selection, is deferred to a companion paper.