page-proposal: Chain Fusion Signer usage guide

[marc0olo]

Proposed page

guides/chain-fusion/chain-fusion-signer.md

What it would cover

• How to use the chain-fusion-signer canister to sign transactions for external chains from web apps and CLI
• ICRC-2 cycle payment model
• Key derivation and address generation for supported chains
• OISY Wallet as a production reference implementation

Where it fits

Under guides/chain-fusion/ alongside the Bitcoin, Ethereum, and Solana guides.

Source material

• https://github.com/dfinity/chain-fusion-signer (README, API docs)
• icskills: wallet-integration, ckbtc, evm-rpc

Context

Discussed in PR #23. The Chain Fusion concept page now mentions the signer in the "Building blocks" section but doesn't have a dedicated how-to guide.