docs: Chain Fusion Signer usage guide

[marc0olo]

Summary

• Adds guides/chain-fusion/chain-fusion-signer.md — a task-oriented guide for using the Chain Fusion Signer canister (grghe-syaaa-aaaar-qabyq-cai)
• Covers: ICRC-2 cycle payment setup, Ethereum/Bitcoin address derivation (online + offline via @dfinity/ic-pub-key), ETH transaction signing, BTC send, generic ECDSA/Schnorr signing, web app integration pattern, fee reference table, and payment variant options
• Also includes infrastructure: three new submodules (chain-fusion-signer v0.3.0, papi v0.1.1, ic-pub-key v1.0.1) with CLAUDE.md/VERSIONS updates and a decisions.md entry
• Closes page-proposal: Chain Fusion Signer usage guide #24

Notes

• All CLI examples use icp canister call (no dfx). The icp canister call command does not support --with-cycles without a proxy, so all examples use the CallerPaysIcrc2Cycles ICRC-2 pre-approval model, which is also the recommended approach for web apps.
• The TypeScript web app snippet is annotated with <!-- Needs human verification --> — the actor creation pattern is correct in principle but should be verified against the actual generated bindings.
• eth_sign_transaction example uses placeholder gas/fee values; real values depend on current network conditions.
• Page uses inline code snippets throughout (no #region markers in .sources/examples for the signer).

Sync recommendation

Informed by dfinity/chain-fusion-signer — src/signer/canister/signer.did, src/signer/api/src/methods.rs, README.md, check-pricing.report.md; dfinity/papi — README.md; dfinity/ic-pub-key — README.md, src/cli.ts

[marc0olo]

can't we link internally to reference/system-canisters/#cycles-ledger ? would be better IMO

[marc0olo]

Feedback addressed:

• Replaced both Learn Hub links to Cycles Ledger with internal links to reference/system-canisters.md#cycles-ledger
• Added sidebar: order: 5 to frontmatter so the page appears after the existing chain guides (Bitcoin=1, Ethereum=2, Solana=3, Dogecoin=4) in the autogenerated navigation

[marc0olo]

Review findings

Verified correct ✅

• Canister ID (grghe-syaaa-aaaar-qabyq-cai) and Cycles Ledger ID (um5iw-rqaaa-aaaaq-qaaba-cai) — confirmed against canister_ids.json
• All API method signatures and Candid argument ordering — confirmed against signer.did
• All fee amounts — confirmed against src/signer/api/src/methods.rs at v0.3.0
• icp canister call Candid syntax for all examples
• Offline CLI flags (-u, -n) — confirmed against src/cli.ts
• All payment type variants — confirmed against signer.did and papi/README.md
• Internal links (system-canisters.md#cycles-ledger, bitcoin.md, ethereum.md)
• <!-- Upstream: --> comment present and accurate
• Build passes cleanly

Fixed in this PR ✅

• icp generate signer.did → npx @icp-sdk/bindgen --did-file signer.did --out-dir — the icp generate command does not exist in icp-cli; replaced with the correct @icp-sdk/bindgen CLI tool

Minor suggestions (non-blocking)

• The fee table version note (at v0.3.0) will become stale on next signer release — consider linking to the source instead of embedding the version label, or noting it's approximate
• "~27 signing operations" comment in the approve section implicitly assumes 37B-cycle methods; a parenthetical clarification would help
• The TypeScript snippet is already flagged with <!-- Needs human verification --> — the createActor import pattern should be verified against actual @icp-sdk/bindgen output once bindings are generated