Docs: Lead performance/cost table with speculative write costs (the default)

[sodre]

Problem

The performance documentation shows both normal path and speculative write costs, but doesn't emphasize that speculative writes are enabled by default. A reader scanning the cost table sees the normal path first and assumes that's the default cost.

This led to a concrete misunderstanding: estimating 10M non-cascade calls at $7.50/month (normal path: 1 RCU + 1 WCU per call) when the actual default cost is ~$6.88/month (speculative: 0 RCU + 1 WCU per call, with ~5% fallback).

Suggested Changes

1. Reorder the cost table to lead with speculative write costs since they are the default behavior.

2. Add a callout at the top of the performance section:

> **Note:** Speculative writes are enabled by default. The costs
> below reflect the default configuration. To use the normal
> read-before-write path, set `speculative_writes=False`.

3. Add a quick-reference cost summary for common scales:

Scale	Non-Cascade (default)	Cascade (default)
1M calls/month	~$0.69	~$1.29
10M calls/month	~$6.88	~$12.85
100M calls/month	~$68.80	~$128.50

(Assuming ~95% speculative hit rate, on-demand pricing)

Why This Matters

Cost estimation is a key factor when evaluating rate limiting solutions. If the docs lead with the more expensive path, users may overestimate costs and choose a different solution unnecessarily.