Attenuation Calculator: polychromatic beams and quasi-monochromatic sources for X-ray CT energy selection

A single-file, browser-based X-ray attenuation calculator for micro-CT, industrial CT, and X-ray imaging design.
Computes monochromatic (2–500 keV) and polychromatic (20–500 kVp) attenuation, optical depth, and transmission for 450+ species, minerals, battery materials, elements, polymers, biological tissues, ceramics, metals, and alloys. Polychromatic mode uses SpekPy v2 (spekcalc) tungsten-anode tube spectra with up to four user-defined filters (Be, Al, Cu, Mo, Sn) and reports effective mu, half-value layer, effective energy, and beam-hardening factor. Custom energy grid for regular-spaced data export. NIST-verified mass attenuation data via xraydb. Runs entirely in the browser, no server or install required.

### Added
- **In-app help mode.** A `?` toggle in the header (on by default; switch it off any time) reveals contextual hints on hover or keyboard focus across the interface: the monochromatic and polychromatic modes, tube voltage, filters, beam source, the three chart tabs (linear attenuation, optical depth, transmission), species selection, and the polychromatic readout.
- **Always-on explanatory badges** on the three most easily-misread metrics, half-value layer (HVL), effective energy, and beam-hardening factor (BHF). These show whether or not help mode is on, because each is a spectrum- and thickness-dependent summary rather than a material constant and warrants a permanent caveat (e.g. HVL is not fixed, the second half-value layer is thicker than the first because the beam hardens with depth).
- Help popovers are themed to the active light / dark / cyberpunk palette, keyboard-accessible (focus to open, Escape to close), and tap-to-toggle on touch devices.

The user interface supports simultaneous comparison of up to 10 species on three physics views (µ, τ, %T), click-to-place reference cursors with log-log interpolation, zoom/pan, and export to PNG (publication-quality) and CSV. The application runs entirely in the browser; no data leaves the user's computer.

Intended audiences: geoscientists, battery researchers, medical physicists, materials scientists, mechanical/aerospace engineers, archaeologists and conservators.