v0.54.6 — chore: VERSION 0.54.6 to match package.json

Added — Gemini 4 Argon price row

google/gemini-4-argon (live 2026-10-20 on Base and Solana) reserves its own
$2/$10 per 1M instead of the $5/$30 default. That is Google's rate through
2026-11-30; it moves to $4/$20 on 2026-12-01. Argon has no long-context tier.
It is priced only: no routing tier or mode includes it.