docs: Chain Fusion concept page

[marc0olo]

Summary

• Explains Chain Fusion vision: cross-chain interoperability without bridges or oracles
• Covers the three building blocks: chain-key signatures, reading external chain state, submitting transactions
• Documents three integration patterns: direct (Bitcoin, Dogecoin), dedicated RPC canister (Ethereum/EVM, Solana), raw HTTPS outcalls (all other chains)
• Includes supported chains table with signature schemes, integration methods, and chain-key tokens
• Covers chain-key tokens (ckBTC, ckETH, ckERC20) as the bridgeless asset model
• Lists building blocks: Bitcoin API, EVM RPC canister, SOL RPC canister, chain-key tokens, Chain Fusion Signer (with OISY Wallet as production example)
• Lists example use cases: trustless cronjobs, multichain wallets, onchain frontends, cross-chain DeFi, decentralized oracles
• Deduplicates the supported chains table from chain-key-cryptography.md — that page now cross-links here instead of maintaining its own copy
• Records three decisions in decisions.md

Related issues

• page-proposal: Chain Fusion Signer usage guide #24 — page-proposal: Chain Fusion Signer usage guide (future work)
• proposal: Move wallet-integration guide from authentication to DeFi section #25 — proposal: Move wallet-integration from authentication to DeFi section (future work)

Sync recommendation

Upstream-informed: dfinity/portal docs/building-apps/chain-fusion/overview.mdx, supported-chains.mdx, evm-rpc/how-it-works.mdx