Browser JPEG/WebP target-size benchmarks (108 headless-Chrome runs)

Measured byte outputs of browser-side canvas.toBlob() encoding for 3 synthetic image classes (photo, screenshot, flat) × 3 source resolutions × 2 formats (JPEG, WebP) × 6 target sizes (20KB–5MB): 108 measurement runs. Each row records the quality and scale parameters a binary search chose, the final output dimensions, and the resulting byte size. Includes a reusable hitTargetSize() tool and the full measurement harness so anyone can reproduce the numbers. Method, environment (headless Chrome 156.0.8078.80, Windows 11) and five stated limitations are documented in METHOD.md. Source repository: github.com/mingyuetutu0527-glitch/exact-size-benchmarks. The same target-size search runs in ExactSize, a browser-only image compressor.