[FEATURE] Max 20x: let fast mode draw from plan usage or the monthly API credits, not usage credits only

Problem

On Max 20x ($200/mo), fast mode on Opus 5.5 is billed to usage credits only. The fast-mode docs say it "draws directly from usage credits, even if you have remaining usage on your plan." So the weekly plan limits and the monthly $200 API credits that now come with Max 20x can both sit unused while every fast-mode token is billed extra at $8/$40 per MTok.

Earlier requests #31880 and #37704 were closed by the stale bot for inactivity, not by a decision, and #41428 was closed as intended behavior. I'm filing again as the bot suggested.

Proposal (any one of these would help)

1. Let fast mode use plan usage at a 2x rate, which matches the price ratio ($8/$40 vs $4/$20 per MTok on Opus 5.5). Bill usage credits only after the weekly limit runs out.
2. Let the monthly API credits included with Max pay for fast mode in Claude Code.
3. Include a small weekly fast-mode allowance in Max 20x.

Why

• Max 20x users pay the most and run the long interactive sessions where speed matters most.
• The current split surprises people: plan usage barely moves while extra charges pile up (see the comments on #31880).

Environment: Claude Code desktop (Windows), Max 20x, Opus 5.5.