feat: add Soroswap swap-then-stealth integration guide

[brooksdevon001-star]

Adds a new integration guide for Soroswap — Stellar's primary AMM and aggregator — showing how
to combine a token swap with a Wraith stealth announcement in a single flow.

What's in the guide:

• Concept diagram walking through the full swap-then-stealth lifecycle
• Soroswap API client setup (typed helpers for /quote, /quote/build, /send)
• Route quoting across Soroswap, Phoenix, and Aqua liquidity sources
• Slippage (slippageBps) and quote freshness handling, with an explicit warning against caching
  quotes
• Building the unsigned transaction with the recipient's stealth address in the to field — the
  key integration point
• Signing, submitting, and announcing via the Wraith Soroban announcer contract
• Complete end-to-end swapThenStealth() function (copy-pasteable, testnet-ready)
• Recipient scanning with fetchAnnouncements + scanAnnouncements
• Error handling with retry logic, protocol selection table, and a production checklist

Nav: New "Integrations" group added to the Guides sidebar with this as the first entry.

CI: All 430 checkable snippets pass pnpm run check:snippets.
closes #80

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[truthixify]

Merged, thanks. Soroswap integration doc has been requested a bunch of times.