docs(assets): document USDC on Base, B20 storage slots, issuer interop

[youssefea]

What changed? Why?

• Adds a "USDC on Base" section to Bridge to Base: native Circle USDC address, how legacy bridged USDbC relates to it, CCTP as the cross-chain path, and Circle EURC addresses.
• Adds a "Read B20 and Policy Registry State" section to Predicates and Safety with the ERC-7201 namespace roots, field offsets, and an allowlist predicate example.
• Adds network status and cross-issuer interoperability (transfers, shared policies, conversion, redemption) to Issue a Stablecoin.
• States that createB20 needs no role or allowlist on Create an Asset Token.
• Adds a Token Standards section to Smart Contracts covering contract-based standards and B20 compatibility.
• Clarifies that the Tokenized Stocks API does not return wallet positions or profit and loss.

Notes to reviewers

Storage offsets come from base/base precompile storage and base-std layout tests, and were spot-checked against mainnet state with cast storage.

How has it been tested?

node scripts/lint-mdx.js on changed files (0 errors), node scripts/validate-docs-structure.js, npm test; external links checked.

Screenshots

Not captured.

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[linear]

DEVREL-818