Function parseAdcpMajorVersion

Extract the major version number from an AdCP version string.

Accepts:

• Semver: '3.0.0', '3.0.1', '3.1.0-beta.1' → 3
• Legacy aliases: 'v3' → 3, 'v2.5' / 'v2.6' → 2

Returns NaN for unrecognized strings — callers should validate before passing.