Spinor — the true object under a spin-½ particle's "spin arrow." Defining trait: it takes 720°, not 360°, to return to itself; one full turn multiplies it by −1.
Found: you asked for it after the positronium build ("add a spinor"). Grounding: Dirac / Pauli spin-½; rotation operator R(2π)=−I for any axis. Real: Rauch 1975 neutron interferometry measured the −1.
LIT720° period, R(2π)=−I, cos(θ/2) phase — node-verified.
AMBERbelt-trick as intuition (illustrative, not a proof).
W3 · phase (1D)—
W4 · arrow (2D)—
W5 · Bloch (3D)—
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