Make scale-reference stateless: settings per call, no global config

[YauhenBichel]

The package kept one shared, changeable settings object: configure() changed it for every caller in the app, and resetConfig() put it back. This PR removes that. Every setting is now passed with the call, and the package keeps no state at all.

What changes

Before	After
configure({maxTiltDegrees, minReferencePixels, references}) changed module state for everyone	fromEllipse(ellipse, diameterMm, options?). The options apply to that call only.
getConfig() / resetConfig()	Removed. DEFAULT_OPTIONS is a frozen constant.
REFERENCES, MAX_TILT_DEGREES, MIN_REFERENCE_PIXELS were getters over the shared object	The same names are plain frozen constants.
A mistyped option (maxTilt) was silently ignored	An unknown or non-positive option throws a TypeError.
explain(reason) had fixed English text	explain(reason, messages?) accepts your own text or translation, with default as a fallback.

The reference table (DEFAULT_REFERENCES) stays as a frozen, published-size constant. Nothing reads it for you; fromEllipse takes any diameter, so apps keep their own references.

The version goes to 0.2.0 because the removed configure, getConfig and resetConfig break the old API. Nothing is published yet. The MoleCare app never calls them: its adapter only re-exports them, and it will be updated when the pin is bumped. The app's own ScaleReference test (17 cases) passes unchanged against this code.

How it was checked

• 25 tests (was 19). All 7 new or changed tests fail on the old code.
• One test fails if any source file declares a module-level let or var, so state cannot creep back in.
• Tarball: 7 files, all expected.
• README and CONTRIBUTING now describe the stateless design.

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