[Usage Transparency] No way to verify '2x usage promo' + Observed quality degradation on Opus 4.6

## Problem

The March 2026 usage promotion claims to double 5-hour usage during off-peak hours. However, multiple users (including myself) have observed:

### 1. Stealth limit reduction before the promo
- Base usage limits appear to have decreased ~50% in the weeks prior to the promotion
- "Doubled" usage during off-peak hours feels identical to pre-reduction normal usage
- The promo may effectively be restoring previously reduced limits, not adding new capacity

### 2. No usage transparency
- There is no counter, API endpoint, or UI indicator to verify actual remaining quota
- Users cannot distinguish between a genuine 2x increase vs. restoration of reduced limits
- This creates a trust problem that is impossible to resolve without data

### 3. Observed quality degradation on Opus 4.6
Since mid-March 2026, I have noticed significant quality drops on Opus 4.6, particularly on complex multi-step reasoning tasks:
- Model takes shortcuts instead of following actual computation logic
- Makes up numbers instead of computing from formulas (e.g., financial modeling tasks)
- "Solves" problems by pattern-matching to closest-looking answer instead of doing the math
- Previously identical tasks were completed perfectly; now they frequently fail
- Multiple Reddit threads corroborate this (r/ClaudeAI, r/ClaudeCode, r/Anthropic — March 17-20, 2026)

This coincides with:
- March 17-18 elevated errors / outages on Opus 4.6
- The off-peak "2x promo" period

### Possible explanations
- Reduced thinking tokens or reasoning depth during high-load periods
- Silent model routing to weaker variants under the Opus 4.6 label
- Infrastructure changes affecting model performance

## Feature Request

Expose a `/v1/usage` or similar endpoint that shows:
- Current 5-hour window remaining tokens
- Whether off-peak multiplier is active
- Historical usage per window
- Model version/variant actually serving requests

This would allow users to verify promotions, track quality, and build trust in the platform.

## Environment
- Plan: Max ($200/month)
- Model: Opus 4.6
- Surfaces: Claude Code, Claude Desktop, API via OpenClaw
- Location: Sydney, Australia (off-peak most of the day)
- Period: March 13-20, 2026