Boundary–Flow–History Coupling with Observer Backaction: Mathematical Structure, Numerical Verification, and Predictive Limits

We examine a candidate coupling of incompressible flow, a degenerate structural field, an internal history variable and heat. Under explicit regularity, positivity and boundary-port assumptions, the model has conditional energy and thermal-entropy balances and strict reductions to a selected positive-sign compact-support comparison equation and the bulk Navier–Stokes equations. The historical cylinder outlet condition is not automatically equivalent to zero symmetric traction. Separately introduced observer extensions R0–R2 exchange power with the fluid or structural field and yield conditional reconstruction formulas, second- and third-order history-to-output delays, and a distinction between apparatus reset and target-memory erasure. Finite model-internal numerical tests and a cross-discretization comparison support selected implementations. The prospectively separated synthetic prediction gates P3.3a, P3.3b and P3.4 FAIL: tested relative-time features and additional simulated heat readings do not meet their predefined improvement thresholds. Ordinary internal-variable viscosity models admit an algebraically identical description. BIG-specific added predictive value is NOT DEMONSTRATED. No general coupled existence/uniqueness or two-dimensional sharp free-boundary theorem, calibrated material law, physical sensor experiment or new universal physical law is established. Publication scope: English primary preprint v1.0, Japanese reference translation/reorganization (not word-for-word), and a combined reproducibility ZIP with all 12 unchanged stage source archives, code, frozen inputs, saved outputs, failures, manifests and local reproduction guides. The release audit checks 12 ZIP CRCs and 458 primary-manifest SHA256 entries; it is not a new PDE simulation or independent third-party rerun. Basically reproducible; independent third-party rerun auditing remains incomplete. Original B42 SCIENCE NONE, RTM2 CP13/CP14 NO_GAIN, Compact Support FAIL, initial implementation deviations and all prediction FAIL outcomes are preserved. Rights: CC BY 4.0 applies to the final papers and new release documentation. It does not relicense archived source code or third-party dependencies. No permissive software license is inferred for historical code lacking an explicit license; see LICENSES.md. No third-party experimental PIV data are redistributed. Historical private Drive links inside original archives are provenance only; included stage reproduction does not require Drive access. This is an unreviewed preprint and a new independent record, not a new version of Fluid History or an earlier B-series paper.