Durable Objects pricing contradiction (Cloudflare billable unit rounding)

### Existing documentation URL(s)

https://developers.cloudflare.com/durable-objects/platform/pricing/

### What changes are you suggesting?

Ensure any pricing explanation in the Docs follows Cloudflare's billable unit rounding to match production calculations.

### Additional information

The Durable Objects pricing examples in the documentation appear to show **proportional billing** for usage above the included allowance rather than rounding usage up to the next billing tier.

For example, the docs use the following calculation:

```text
Compute Duration:

100 Durable Objects × 1 second × 60 minutes × 24 hours × 30 days = 4,320,000 seconds
4,320,000 seconds × 128 MB / 1 GB = 552,960 GB-s
(552,960 GB-s - included 400,000 GB-s) × $12.50 / 1,000,000 = $1.91
```

This indicates that the overage of **152,960 GB-s** is billed proportionally:

```text
152,960 / 1,000,000 × $12.50 = $1.91
```

If billing instead rounded up to the next full million GB-s tier, the calculation would be:

```text
ceil((552,960 GB-s - included 400,000 GB-s) / 1,000,000) × $12.50

= ceil(152,960 / 1,000,000) × $12.50

= 1 × $12.50

= $12.50
```

So the documentation example suggests that Durable Objects charges are metered and billed proportionally, rather than rounded up to the next million GB-s increment.

Start with the Durable Objects pricing page at https://developers.cloudflare.com/durable-objects/platform/pricing/ and compare its examples with Cloudflare's billable unit rounding used in production calculations. Confirm the correct treatment of usage above the included allowance, then update the pricing explanation and example so the documented result matches billing behavior.

Written by the indexing model from the issue text.