Add Dfns to the wallet directory

[adriendfns]

Type of submission

• New wallet application

Summary

Adds Dfns (institutional wallet-as-a-service platform) to the wallet
directory, self-attesting Canton Coin support, CC pre-approvals, CIP-0056
transfers, WalletConnect, memo tags, the Wallet Gateway signing driver,
clear signing, and enterprise wallet capabilities (MPC/TSS and HSM key
generation, policy workflows, key recovery, validator hosting incl. BYOV).

Checklist

• wallets/dfns.yaml added (self-attestation)
• A heading added to proofs/dfns/dfns-self-attestation.md
  for every feature newly claimed as supported: true
• python scripts/generate_table.py was run and the regenerated
  WALLET_DIRECTORY.md is included in this PR
• Applying for the first time: confirmed membership in
  #ext-temp-party-scaling and set slack_party_scaling_channel_joined: true
• Marking a feature as not supported: registry_preapprovals is marked
  supported: false with a reason ("Not supported yet")

Additional context

Evidence in the self-attestation currently points to Dfns's public
announcements and documentation (Canton Tier-1 support, CIP-56 token
standard support, Canton Wallet Gateway signing driver, WalletConnect
guide, transfer endpoint memo field). We're happy to follow up with
on-chain transaction update IDs and screenshots for any claim on request.

🤖 Generated with Claude Code