[mikemaccana]

Follows quicknode/solana-program-examples#172, which removed set_funding_rate(). Before that change, Maria could open a small position on the side that receives funding, raise the rate, and close the position to take the liquidity providers' deposits. initialize_pool() now refuses a rate above MAX_FUNDING_RATE_PER_SECOND (277, just under 0.1% of a position's size per hour), and nothing can change the rate afterwards.

This PR was stacked on #190. It now targets main, and main has been merged in.

Changes to the Perpetual Futures chapter

• Creating the market: the funding rate is set in initialize_pool(), and no handler can change it once the pool exists.
• Funding section:
  • The set_funding_rate() sentence is replaced by the fixed, capped rate.
  • It says what Maria could have done if she could change the rate.
  • It names test_operator_on_the_lighter_side_earns_only_the_fixed_rate.
• Funding example: it now uses the maximum rate, so it reads 2.77 USDC over 2,000 seconds instead of 50 USDC at a rate of 5,000. The funding-index figure changes to match.
• "The payment goes to the pool" said the lighter side isn't paid. In fact the pool pays the lighter side at the same rate, out of liquidity, and the bullet now says so.
• Maria's summary: collect_fees() is again her only signed handler once the pool exists.
• Share-inflation paragraph: it uses the test's new figures. The attacker holds a long for ten simulated years at the maximum rate and pumps AUM to about 88,000 minor units. They get back 88 of about 87,000 paid.

npm run lint, node scripts/check-figure-geometry.mjs and npm run build-pdf all pass. Every identifier named exists on the companion repo's main.

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