docs: [lend] add Lend AMM integration guide for routers

[0xYankee-Raccoons]

Summary

Ports Fluid's DEX swap integration guide into the docs as a public Lend AMM section under Lend, for routers, aggregators, and on-chain programs that want to route swaps directly through Jupiter Lend as a DEX (Jupiter's own routing engine already integrates it). Source is the DEX docs on the dex-deployments branch of the private Instadapp/fluid-contracts-solana repo; because that repo is private, the pages are fully self-contained with no links back to it.

Naming convention (per YY): product is the Lend AMM; "DEX" is reserved for the on-chain program (jupZ4m2…) and code identifiers (Dex, dex_id, DexOperation, Dex* errors). File paths stay at lend/dex/ to match the program name and keep preview links stable.

Changes

Nav: new "Lend AMM" group in the Lend sidebar, ordered Earn → Borrow → Lend AMM.

• lend/dex/index.mdx — AMM Overview: pool shapes, swap_in/swap_out, integrator conventions, program addresses, PDA seeds, plus a condensed architecture section (Liquidity Layer relationship, shares, re-entrancy, pausing) with a Mermaid CPI-flow diagram. Consolidated from two source pages (integration README + architecture).
• lend/dex/typescript.mdx — AMM SDK: quote → guard → execute with @jup-ag/lend-read + @jup-ag/lend/dex, ALT/versioned-transaction handling, simulation-based exact quoting, error handling.
• lend/dex/api.mdx — AMM APIs (coming soon): stub pointing at the SDK and CPI paths until the REST endpoints launch.
• lend/dex/cpi.mdx — AMM CPI Integration: declare_program! CPI client, the 25-account DexOperation context, PDA signing, external center-price remaining accounts, gotchas, raw instruction recipe for non-Anchor programs.
• lend/dex/errors.mdx — AMM Errors: full swap-path error code reference.
• lend/program-addresses.mdx — added the AMM/DEX program.
• changelog/index.mdx — July 2026 entry.
• .claude/rules/ — decisions entry, product-learning section, style-guide naming + Mermaid conventions.

Deviations from the upstream doc (verified against npm/mainnet)

• Upstream claims the IDL ships at @jup-ag/lend/dist/idl/dex.json — it does not (checked 0.1.10 and 0.2.0-beta.2 tarballs). The IDL is published on-chain, so the CPI page uses anchor idl fetch instead.
• The ./dex subpath (getSwapInIx/getSwapOutIx) only exists on the beta dist-tag (0.2.0-beta.2), not latest (0.1.10). The SDK page carries a Note about installing @jup-ag/lend@beta until 0.2.x ships to latest — drop the Note at launch.

Linear Issues

• Fixes DEV-690 — Lend dex integration guide

Checklist

• node generate-llms-from-docs.js run
• mint broken-links passes
• All pages have title, description, llmsDescription
• docs.json navigation updated
• Redirects added (n/a — new paths only)
• Changelog entry added to changelog
• .claude/rules/ updated with learnings and IA decision

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