docs(b20): explain how stablecoins from different issuers interoperate

[youssefea]

What changed? Why?

Adds an "Integrate Stablecoins From Other Issuers" section to docs/get-started/issue-stablecoins.mdx. It explains what B20 stablecoins from different issuers have in common (the ERC-20 surface and currency()) and what stays specific to each token: its transfer policies, how shared Policy Registry lists work, the fact that B20 has no conversion function, and the fact that redemption happens with each token's own issuer.

This closes Base Knowledge Base gap gap-20261006-b20-stablecoin-interoperability-across-issuers: "How do independently issued B20 stablecoins (e.g. from different banks) interoperate on Base for transfers, conversion, and redemption, given each issuer has its own policies?"

• Gaps log: https://bdocs.cbhq.net/d/base-knowledge-gaps-log
• Linear: DEVREL-859

Notes to reviewers

This PR replaces the part of #2093 that covered this gap. Compared with #2093, it drops the guidance to convert "through an onchain swap or a venue" and the claim that code "works for any other" issuer's token. The section does not recommend any conversion venue or promise any redemption model across issuers.

Primary sources, each a page already on master:

• Shared ERC-20 surface: docs/specifications/b20/introduction.mdx (line 68), docs/specifications/b20/concepts/token-types.mdx
• currency(): docs/specifications/b20/reference/interfaces/ib20-stablecoin/index.mdx
• Each token's policies cover the sender, receiver, and executor; isAuthorized; many tokens can share one policy: docs/specifications/b20/concepts/policies.mdx (§1, §2.1, §3.2), docs/build-on-base/issue-stablecoins/restrict-who-can-hold.mdx
• Redemption means the holder returns tokens and the issuer burns them: docs/build-on-base/issue-stablecoins/burn-supply.mdx

Needs SME confirmation: the B20 spec owners should confirm two statements before merge:

1. "B20 does not define a function that converts one issuer's stablecoin into another's."
2. "B20 does not define redemption of one issuer's token by another issuer."

Both are based on the interface reference, which lists no such function. If an endorsed model for conversion or redemption between issuers exists, the owners should say whether it should be documented here.

How has it been tested?

• node scripts/lint-mdx.js --files-from=<changed file list>: 0 errors, 0 warnings.
• node scripts/validate-docs-structure.js: fails with 2 errors about the /upgrades/denim/eip-8130 redirect. Those errors are already on master and this PR does not cause them.
• Checked that every internal link resolves to an existing .mdx file.

Screenshots

N/A (text-only change)

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[cb-heimdall]

🟡 Heimdall Review Status

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[linear]

DEVREL-859